"L Alpert" <alpertl@xxgmail.com> wrote in message
news:sHTMg.17447$1f6.2521@newssvr27.news.prodigy.n et...
> Michael Pardee wrote:
>> "L Alpert" <alpertl@xxgmail.com> wrote in message
>> news:mZxMg.23979$kO3.6534@newssvr12.news.prodigy.c om...
>>>
>>> I don't believe the original gas mileage matters, only the
>>> differences. Maybe one can assume that windows open would be less of an
>>> effect on
>>> a vehicle with more mass and available torque (unless the vehicle
>>> allowed a higher volume of air)?
>>>
>> Vehicles with lower fuel economy are less sensitive to differences,
>> because the losses are already pretty high. In the Toyota Prius fora
>> people are shocked to find that running the heater can decrease the
>> in-town fuel economy 10 mpg... the finer the edge the more quickly it
>> dulls.
>> Mike
>
> The raw number would be lower, but the percentages should be similar.
>
I think the discrepancy comes from working with the reciprocal of what we
really want to measure: fuel per increment. For example, suppose it requires
5 gallons of gas to move the car 100 miles with the windows rolled up and
A/C off. That is 20 mpg. If the window drag at some speed consumes 0.2
gallons in 100 miles at that speed the economy drops to 19.2 mpg for a loss
of nearly 5%. If the drag is the same in a vehicle that requires 2 gallons
to move the car 100 miles (50 mpg) the window drag drops it to 45.5 mpg, a
nearly 10% hit. I'm not a big fan of the "liters per 100km" measurement but
it works a lot better than mpg here.

So, here's an example of how it goes - theoretically - with the window and
A/C. Assume (for illustration) that at 50 mph the window drag consumes 0.1
gallon per hundred miles. Similarly, assume the A/C consumes 0.1 gallon per
hour. For our hundred mile trip that would mean the windows would use 0.1
gallon and the A/C would use 0.2 gallons at 50 mph. If we increase the speed
to 100 mph the window drag, increasing with the square of the speed, becomes
0.4 gallons for the one hour the trip takes while the A/C loss drops to 0.1
gallon for the hour instead of 0.2 gallons for two hours.

For different vehicles the numbers would change; the A/C losses are higher
for a larger vehicle with more glass and the window drag will certainly vary
with body style. In the end, all that will change is the speed at which the
A/C is more economical than windows down.

 

Gordon McGrew wrote:
> On Sat, 09 Sep 2006 11:43:49 GMT, "L Alpert" <alpertl@xxgmail.com>
> wrote:
>
>> JXStern wrote:
>>> On Wed, 06 Sep 2006 19:41:47 GMT, gRmEcMgOrVeEw@mindspring.com
>>> (Gordon McGrew) wrote:
>>>> It has become popular lately to claim that turning on A/C uses less
>>>> fuel than opening windows. On a recent round trip to Kansas I used
>>>> my handy Scan Gauge to check milage under different operating
>>>> conditions. (What else is there to do driving across Missouri and
>>>> Kansas?)
>>> ...
>>>
>>> You sir are a gentleman and a scholar.
>>>
>>> However, the numbers are small enough, say 2%, that on a $50 tank of
>>> gas we're talking $1.00. And you sure can't hear the radio with the
>>> windows open more than a crack at speed.
>>>
>>> I do guess the window numbers would be more significant for an
>>> Accord without the antlers and big slab sides, might get it up to,
>>> oh, who knows, 5%?!? But presumably the A/C numbers would be the
>>> same, which would validate the claim which your experiment did not,
>>> but would still be just about $1.00/tank in the other direction.
>>>
>>> OTOH, windows would impose less overhead at slow speeds, say in stop
>>> and go, ... oh, where will it end?!
>>>
>>> J.
>>
>>
>> It is good information, but the true differences are difficult to
>> assess, as many segments are averaged together, so one cannot
>> calculate the standard deviations for each group. It does seem that
>> on 9/7 there was less head wind.....
>
> Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5.
>
> The behavior of the Scan Gauge is a little flaky in regards to
> measuring fuel consumption. I recalibrate it at every fuel stop and
> it can vary by 10% or more. Since the calibration changed between
> test sessions, you can't compare the absolute milage figures between
> sessions. Also, gradually changing terrain, elevation, wind and
> different test speeds complicate the situation. That is why I looked
> at relative fuel economy between the sessions. Below is the
> individual session data in case you or someone else wants to do more
> sophisticated analysis. Just keep the above in mind.
>
> Test 1 8/31 76 mph
>
> A/C Window m/g miles
> on closed 18.2 10.0
> off closed 19.5 10.0
> off wide opn 20.2 10.0
> off closed 21.2 10.0
> on closed 17.4 10.0
> off closed 18.9 10.0
> off wide opn 18.7 10.0
> on closed 18.8 10.5
> off closed 19.7 10.0
> off F3 R3* 19.3 10.0
> on closed 19.1 10.0
> off R6 18.5 10.0
>
>
> * F3 R3 = front window down 3", Rear window down 3"
>
>
> Test 2 9/4 75 mph
>
> A/C Window m/g miles
> off closed 21.0 10.0
> off R4 20.0 10.0
> off closed 22.5 8.7
> on closed 20.9 10.0
> off closed 22.4 11.4
> off F2 R4 22.4 10.0
> off F2 R4 21.4 11.4
> off closed 21.7 10.0
> off closed 21.0 10.4
>
> Test 3 9/5 75 mph
>
> A/C Window m/g miles
> off closed 19.6 10.0
> off R4 20.5 10.0
> on closed 18.5 7.8
> off closed 20.0 10.0
> on closed 19.2 10.2
> off R4 20.8 10.1
> off closed 21.7 10.0
> on closed 20.1 10.0
>
> Test 4 9/5 73 mph flatland
>
> A/C Window m/g miles
> off closed 21.6 10.0
> off F3 R4 21.1 10.2
> on closed 21.2 10.0
> off closed 22.5 10.0
> off F2 R4 22.4 10.9
> on closed 20.6 11.6
> off closed 21.4 10.0
> off F3 R4 21.7 10.0
> on closed 20.6 10.0
> off closed 22.8 13.7
> off F3 R4 21.4 10.1
> on closed 20.3 10.5
> off closed 21.6 12.0
> off F3 R4 21.8 11.0
> on closed 21.4 10.0
> off closed 23.4 13.0

My bad on the dates. 7.2% loss between air on and air off with windows
closed, with almost identical standard deviations with decent sample
sizes.

It looks like with windows partially open is just about the same as them
closed (.4% difference probably insignificant due to standard test
error) until you get to the 6" opening in the rear (though there is only
1 sample), which is just about the same as with them wide open (again,
small sample size). 6" to wide open the same as with the air on and
closed.

Probably could use more "open window" numbers just to verify, but it
makes sense if you look at some simple HVAC calculations for orifice
flow....

A 14" diameter that has about 154 sq/in of area (semi close guess to a
car window, I haven't actually measured one) with a round edge discharge
co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and
almost 100K CFM at 20 psi (sharp edge drops numbers by 40%).
I'm not sure about the effects of the angle of the flow on the opening
and I haven't measured the air pressure entering a car through an open
window moving at 70 mph, but I'm sure it would be quite high (and then
multiple the CFM x 4!) That is a lot of drag......

I'll take the 7.2% loss without the wind howling in my ears!

Thanks for the clarifications. Good real world application.

 

Gordon McGrew wrote:
> On Sat, 09 Sep 2006 11:43:49 GMT, "L Alpert" <alpertl@xxgmail.com>
> wrote:
>
>> JXStern wrote:
>>> On Wed, 06 Sep 2006 19:41:47 GMT, gRmEcMgOrVeEw@mindspring.com
>>> (Gordon McGrew) wrote:
>>>> It has become popular lately to claim that turning on A/C uses less
>>>> fuel than opening windows. On a recent round trip to Kansas I used
>>>> my handy Scan Gauge to check milage under different operating
>>>> conditions. (What else is there to do driving across Missouri and
>>>> Kansas?)
>>> ...
>>>
>>> You sir are a gentleman and a scholar.
>>>
>>> However, the numbers are small enough, say 2%, that on a $50 tank of
>>> gas we're talking $1.00. And you sure can't hear the radio with the
>>> windows open more than a crack at speed.
>>>
>>> I do guess the window numbers would be more significant for an
>>> Accord without the antlers and big slab sides, might get it up to,
>>> oh, who knows, 5%?!? But presumably the A/C numbers would be the
>>> same, which would validate the claim which your experiment did not,
>>> but would still be just about $1.00/tank in the other direction.
>>>
>>> OTOH, windows would impose less overhead at slow speeds, say in stop
>>> and go, ... oh, where will it end?!
>>>
>>> J.
>>
>>
>> It is good information, but the true differences are difficult to
>> assess, as many segments are averaged together, so one cannot
>> calculate the standard deviations for each group. It does seem that
>> on 9/7 there was less head wind.....
>
> Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5.
>
> The behavior of the Scan Gauge is a little flaky in regards to
> measuring fuel consumption. I recalibrate it at every fuel stop and
> it can vary by 10% or more. Since the calibration changed between
> test sessions, you can't compare the absolute milage figures between
> sessions. Also, gradually changing terrain, elevation, wind and
> different test speeds complicate the situation. That is why I looked
> at relative fuel economy between the sessions. Below is the
> individual session data in case you or someone else wants to do more
> sophisticated analysis. Just keep the above in mind.
>
> Test 1 8/31 76 mph
>
> A/C Window m/g miles
> on closed 18.2 10.0
> off closed 19.5 10.0
> off wide opn 20.2 10.0
> off closed 21.2 10.0
> on closed 17.4 10.0
> off closed 18.9 10.0
> off wide opn 18.7 10.0
> on closed 18.8 10.5
> off closed 19.7 10.0
> off F3 R3* 19.3 10.0
> on closed 19.1 10.0
> off R6 18.5 10.0
>
>
> * F3 R3 = front window down 3", Rear window down 3"
>
>
> Test 2 9/4 75 mph
>
> A/C Window m/g miles
> off closed 21.0 10.0
> off R4 20.0 10.0
> off closed 22.5 8.7
> on closed 20.9 10.0
> off closed 22.4 11.4
> off F2 R4 22.4 10.0
> off F2 R4 21.4 11.4
> off closed 21.7 10.0
> off closed 21.0 10.4
>
> Test 3 9/5 75 mph
>
> A/C Window m/g miles
> off closed 19.6 10.0
> off R4 20.5 10.0
> on closed 18.5 7.8
> off closed 20.0 10.0
> on closed 19.2 10.2
> off R4 20.8 10.1
> off closed 21.7 10.0
> on closed 20.1 10.0
>
> Test 4 9/5 73 mph flatland
>
> A/C Window m/g miles
> off closed 21.6 10.0
> off F3 R4 21.1 10.2
> on closed 21.2 10.0
> off closed 22.5 10.0
> off F2 R4 22.4 10.9
> on closed 20.6 11.6
> off closed 21.4 10.0
> off F3 R4 21.7 10.0
> on closed 20.6 10.0
> off closed 22.8 13.7
> off F3 R4 21.4 10.1
> on closed 20.3 10.5
> off closed 21.6 12.0
> off F3 R4 21.8 11.0
> on closed 21.4 10.0
> off closed 23.4 13.0

My bad on the dates. 7.2% loss between air on and air off with windows
closed, with almost identical standard deviations with decent sample
sizes.

It looks like with windows partially open is just about the same as them
closed (.4% difference probably insignificant due to standard test
error) until you get to the 6" opening in the rear (though there is only
1 sample), which is just about the same as with them wide open (again,
small sample size). 6" to wide open the same as with the air on and
closed.

Probably could use more "open window" numbers just to verify, but it
makes sense if you look at some simple HVAC calculations for orifice
flow....

A 14" diameter that has about 154 sq/in of area (semi close guess to a
car window, I haven't actually measured one) with a round edge discharge
co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and
almost 100K CFM at 20 psi (sharp edge drops numbers by 40%).
I'm not sure about the effects of the angle of the flow on the opening
and I haven't measured the air pressure entering a car through an open
window moving at 70 mph, but I'm sure it would be quite high (and then
multiple the CFM x 4!) That is a lot of drag......

I'll take the 7.2% loss without the wind howling in my ears!

Thanks for the clarifications. Good real world application.

 

Gordon McGrew wrote:
> On Sat, 09 Sep 2006 11:43:49 GMT, "L Alpert" <alpertl@xxgmail.com>
> wrote:
>
>> JXStern wrote:
>>> On Wed, 06 Sep 2006 19:41:47 GMT, gRmEcMgOrVeEw@mindspring.com
>>> (Gordon McGrew) wrote:
>>>> It has become popular lately to claim that turning on A/C uses less
>>>> fuel than opening windows. On a recent round trip to Kansas I used
>>>> my handy Scan Gauge to check milage under different operating
>>>> conditions. (What else is there to do driving across Missouri and
>>>> Kansas?)
>>> ...
>>>
>>> You sir are a gentleman and a scholar.
>>>
>>> However, the numbers are small enough, say 2%, that on a $50 tank of
>>> gas we're talking $1.00. And you sure can't hear the radio with the
>>> windows open more than a crack at speed.
>>>
>>> I do guess the window numbers would be more significant for an
>>> Accord without the antlers and big slab sides, might get it up to,
>>> oh, who knows, 5%?!? But presumably the A/C numbers would be the
>>> same, which would validate the claim which your experiment did not,
>>> but would still be just about $1.00/tank in the other direction.
>>>
>>> OTOH, windows would impose less overhead at slow speeds, say in stop
>>> and go, ... oh, where will it end?!
>>>
>>> J.
>>
>>
>> It is good information, but the true differences are difficult to
>> assess, as many segments are averaged together, so one cannot
>> calculate the standard deviations for each group. It does seem that
>> on 9/7 there was less head wind.....
>
> Better check the date. Tests were run on 8/31, 9/4 and (mostly) 9/5.
>
> The behavior of the Scan Gauge is a little flaky in regards to
> measuring fuel consumption. I recalibrate it at every fuel stop and
> it can vary by 10% or more. Since the calibration changed between
> test sessions, you can't compare the absolute milage figures between
> sessions. Also, gradually changing terrain, elevation, wind and
> different test speeds complicate the situation. That is why I looked
> at relative fuel economy between the sessions. Below is the
> individual session data in case you or someone else wants to do more
> sophisticated analysis. Just keep the above in mind.
>
> Test 1 8/31 76 mph
>
> A/C Window m/g miles
> on closed 18.2 10.0
> off closed 19.5 10.0
> off wide opn 20.2 10.0
> off closed 21.2 10.0
> on closed 17.4 10.0
> off closed 18.9 10.0
> off wide opn 18.7 10.0
> on closed 18.8 10.5
> off closed 19.7 10.0
> off F3 R3* 19.3 10.0
> on closed 19.1 10.0
> off R6 18.5 10.0
>
>
> * F3 R3 = front window down 3", Rear window down 3"
>
>
> Test 2 9/4 75 mph
>
> A/C Window m/g miles
> off closed 21.0 10.0
> off R4 20.0 10.0
> off closed 22.5 8.7
> on closed 20.9 10.0
> off closed 22.4 11.4
> off F2 R4 22.4 10.0
> off F2 R4 21.4 11.4
> off closed 21.7 10.0
> off closed 21.0 10.4
>
> Test 3 9/5 75 mph
>
> A/C Window m/g miles
> off closed 19.6 10.0
> off R4 20.5 10.0
> on closed 18.5 7.8
> off closed 20.0 10.0
> on closed 19.2 10.2
> off R4 20.8 10.1
> off closed 21.7 10.0
> on closed 20.1 10.0
>
> Test 4 9/5 73 mph flatland
>
> A/C Window m/g miles
> off closed 21.6 10.0
> off F3 R4 21.1 10.2
> on closed 21.2 10.0
> off closed 22.5 10.0
> off F2 R4 22.4 10.9
> on closed 20.6 11.6
> off closed 21.4 10.0
> off F3 R4 21.7 10.0
> on closed 20.6 10.0
> off closed 22.8 13.7
> off F3 R4 21.4 10.1
> on closed 20.3 10.5
> off closed 21.6 12.0
> off F3 R4 21.8 11.0
> on closed 21.4 10.0
> off closed 23.4 13.0

My bad on the dates. 7.2% loss between air on and air off with windows
closed, with almost identical standard deviations with decent sample
sizes.

It looks like with windows partially open is just about the same as them
closed (.4% difference probably insignificant due to standard test
error) until you get to the 6" opening in the rear (though there is only
1 sample), which is just about the same as with them wide open (again,
small sample size). 6" to wide open the same as with the air on and
closed.

Probably could use more "open window" numbers just to verify, but it
makes sense if you look at some simple HVAC calculations for orifice
flow....

A 14" diameter that has about 154 sq/in of area (semi close guess to a
car window, I haven't actually measured one) with a round edge discharge
co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and
almost 100K CFM at 20 psi (sharp edge drops numbers by 40%).
I'm not sure about the effects of the angle of the flow on the opening
and I haven't measured the air pressure entering a car through an open
window moving at 70 mph, but I'm sure it would be quite high (and then
multiple the CFM x 4!) That is a lot of drag......

I'll take the 7.2% loss without the wind howling in my ears!

Thanks for the clarifications. Good real world application.

 

Michael Pardee wrote:
> "L Alpert" <alpertl@xxgmail.com> wrote in message
> news:sHTMg.17447$1f6.2521@newssvr27.news.prodigy.n et...
>> Michael Pardee wrote:
>>> "L Alpert" <alpertl@xxgmail.com> wrote in message
>>> news:mZxMg.23979$kO3.6534@newssvr12.news.prodigy.c om...
>>>>
>>>> I don't believe the original gas mileage matters, only the
>>>> differences. Maybe one can assume that windows open would be less
>>>> of an effect on
>>>> a vehicle with more mass and available torque (unless the vehicle
>>>> allowed a higher volume of air)?
>>>>
>>> Vehicles with lower fuel economy are less sensitive to differences,
>>> because the losses are already pretty high. In the Toyota Prius fora
>>> people are shocked to find that running the heater can decrease the
>>> in-town fuel economy 10 mpg... the finer the edge the more quickly
>>> it dulls.
>>> Mike
>>
>> The raw number would be lower, but the percentages should be similar.
>>
> I think the discrepancy comes from working with the reciprocal of
> what we really want to measure: fuel per increment. For example,
> suppose it requires 5 gallons of gas to move the car 100 miles with
> the windows rolled up and A/C off. That is 20 mpg. If the window drag
> at some speed consumes 0.2 gallons in 100 miles at that speed the
> economy drops to 19.2 mpg for a loss of nearly 5%. If the drag is the
> same in a vehicle that requires 2 gallons to move the car 100 miles
> (50 mpg) the window drag drops it to 45.5 mpg, a nearly 10% hit. I'm
> not a big fan of the "liters per 100km" measurement but it works a
> lot better than mpg here.
> So, here's an example of how it goes - theoretically - with the
> window and A/C. Assume (for illustration) that at 50 mph the window
> drag consumes 0.1 gallon per hundred miles. Similarly, assume the A/C
> consumes 0.1 gallon per hour. For our hundred mile trip that would
> mean the windows would use 0.1 gallon and the A/C would use 0.2
> gallons at 50 mph. If we increase the speed to 100 mph the window
> drag, increasing with the square of the speed, becomes 0.4 gallons for the
> one hour the trip takes while the A/C loss drops
> to 0.1 gallon for the hour instead of 0.2 gallons for two hours.
>
> For different vehicles the numbers would change; the A/C losses are
> higher for a larger vehicle with more glass and the window drag will
> certainly vary with body style. In the end, all that will change is
> the speed at which the A/C is more economical than windows down.


The net effect will be determined by environmental conditions and choice.
My preferences are usually determined by speed of travel and
temperature/humidity conditions. <90° and lower humidity while driving on
back roads at <50 mph would be open windows for me for the most part
(general rule of thumb). >90° would most likely be AC all the time.

Highway speeds would either be AC or vent, depending on the preferred
comfort level at that specific time. I am not very concerned about the
differences in fuel economy, only in comfort level (although it is always
nice to know.....).

 

Michael Pardee wrote:
> "L Alpert" <alpertl@xxgmail.com> wrote in message
> news:sHTMg.17447$1f6.2521@newssvr27.news.prodigy.n et...
>> Michael Pardee wrote:
>>> "L Alpert" <alpertl@xxgmail.com> wrote in message
>>> news:mZxMg.23979$kO3.6534@newssvr12.news.prodigy.c om...
>>>>
>>>> I don't believe the original gas mileage matters, only the
>>>> differences. Maybe one can assume that windows open would be less
>>>> of an effect on
>>>> a vehicle with more mass and available torque (unless the vehicle
>>>> allowed a higher volume of air)?
>>>>
>>> Vehicles with lower fuel economy are less sensitive to differences,
>>> because the losses are already pretty high. In the Toyota Prius fora
>>> people are shocked to find that running the heater can decrease the
>>> in-town fuel economy 10 mpg... the finer the edge the more quickly
>>> it dulls.
>>> Mike
>>
>> The raw number would be lower, but the percentages should be similar.
>>
> I think the discrepancy comes from working with the reciprocal of
> what we really want to measure: fuel per increment. For example,
> suppose it requires 5 gallons of gas to move the car 100 miles with
> the windows rolled up and A/C off. That is 20 mpg. If the window drag
> at some speed consumes 0.2 gallons in 100 miles at that speed the
> economy drops to 19.2 mpg for a loss of nearly 5%. If the drag is the
> same in a vehicle that requires 2 gallons to move the car 100 miles
> (50 mpg) the window drag drops it to 45.5 mpg, a nearly 10% hit. I'm
> not a big fan of the "liters per 100km" measurement but it works a
> lot better than mpg here.
> So, here's an example of how it goes - theoretically - with the
> window and A/C. Assume (for illustration) that at 50 mph the window
> drag consumes 0.1 gallon per hundred miles. Similarly, assume the A/C
> consumes 0.1 gallon per hour. For our hundred mile trip that would
> mean the windows would use 0.1 gallon and the A/C would use 0.2
> gallons at 50 mph. If we increase the speed to 100 mph the window
> drag, increasing with the square of the speed, becomes 0.4 gallons for the
> one hour the trip takes while the A/C loss drops
> to 0.1 gallon for the hour instead of 0.2 gallons for two hours.
>
> For different vehicles the numbers would change; the A/C losses are
> higher for a larger vehicle with more glass and the window drag will
> certainly vary with body style. In the end, all that will change is
> the speed at which the A/C is more economical than windows down.


The net effect will be determined by environmental conditions and choice.
My preferences are usually determined by speed of travel and
temperature/humidity conditions. <90° and lower humidity while driving on
back roads at <50 mph would be open windows for me for the most part
(general rule of thumb). >90° would most likely be AC all the time.

Highway speeds would either be AC or vent, depending on the preferred
comfort level at that specific time. I am not very concerned about the
differences in fuel economy, only in comfort level (although it is always
nice to know.....).

 

Michael Pardee wrote:
> "L Alpert" <alpertl@xxgmail.com> wrote in message
> news:sHTMg.17447$1f6.2521@newssvr27.news.prodigy.n et...
>> Michael Pardee wrote:
>>> "L Alpert" <alpertl@xxgmail.com> wrote in message
>>> news:mZxMg.23979$kO3.6534@newssvr12.news.prodigy.c om...
>>>>
>>>> I don't believe the original gas mileage matters, only the
>>>> differences. Maybe one can assume that windows open would be less
>>>> of an effect on
>>>> a vehicle with more mass and available torque (unless the vehicle
>>>> allowed a higher volume of air)?
>>>>
>>> Vehicles with lower fuel economy are less sensitive to differences,
>>> because the losses are already pretty high. In the Toyota Prius fora
>>> people are shocked to find that running the heater can decrease the
>>> in-town fuel economy 10 mpg... the finer the edge the more quickly
>>> it dulls.
>>> Mike
>>
>> The raw number would be lower, but the percentages should be similar.
>>
> I think the discrepancy comes from working with the reciprocal of
> what we really want to measure: fuel per increment. For example,
> suppose it requires 5 gallons of gas to move the car 100 miles with
> the windows rolled up and A/C off. That is 20 mpg. If the window drag
> at some speed consumes 0.2 gallons in 100 miles at that speed the
> economy drops to 19.2 mpg for a loss of nearly 5%. If the drag is the
> same in a vehicle that requires 2 gallons to move the car 100 miles
> (50 mpg) the window drag drops it to 45.5 mpg, a nearly 10% hit. I'm
> not a big fan of the "liters per 100km" measurement but it works a
> lot better than mpg here.
> So, here's an example of how it goes - theoretically - with the
> window and A/C. Assume (for illustration) that at 50 mph the window
> drag consumes 0.1 gallon per hundred miles. Similarly, assume the A/C
> consumes 0.1 gallon per hour. For our hundred mile trip that would
> mean the windows would use 0.1 gallon and the A/C would use 0.2
> gallons at 50 mph. If we increase the speed to 100 mph the window
> drag, increasing with the square of the speed, becomes 0.4 gallons for the
> one hour the trip takes while the A/C loss drops
> to 0.1 gallon for the hour instead of 0.2 gallons for two hours.
>
> For different vehicles the numbers would change; the A/C losses are
> higher for a larger vehicle with more glass and the window drag will
> certainly vary with body style. In the end, all that will change is
> the speed at which the A/C is more economical than windows down.


The net effect will be determined by environmental conditions and choice.
My preferences are usually determined by speed of travel and
temperature/humidity conditions. <90° and lower humidity while driving on
back roads at <50 mph would be open windows for me for the most part
(general rule of thumb). >90° would most likely be AC all the time.

Highway speeds would either be AC or vent, depending on the preferred
comfort level at that specific time. I am not very concerned about the
differences in fuel economy, only in comfort level (although it is always
nice to know.....).

 

"L Alpert" <alpertl@xxgmail.com> wrote
> Good real world application.

Ditto.

What Gordon found appears to be consistent with some study
(or a summary of a study) I read not too long ago. Tom and
Ray of "Car Talk" may have cited it at their web site, if
memory serves. Namely, even when moving at highway speeds,
not using the A/C and leaving windows open for some cooling
is best.

Plenty of studies on the net on this. E.g.
---
Several sources claim that closing windows and using a car's
air conditioner will provide better fuel economy at freeway
speeds than leaving the windows open without the air
conditioner. However, FSEC tests showed this in not the
case. In repeated evaluation at 65 miles per hour, our test
car experienced 11% better fuel efficiency with no A/C and
the windows open than using the air conditioner. We also
found that closing windows at freeway speeds improved fuel
efficiency by 2-3%.

http://www.fsec.ucf.edu/Pubs/energynotes/en-19.htm
---

The above site has a nice chart for freeway speed driving.
The test vehicle was a 1986 VW GTI.

OTOH, the damned people at FSEC.UCF appear to misquote a
study done by edmunds.com, indicating that there's no
difference at highway speeds.

I suspect this all depends on the model and its wind
resistance characteristics more than we'd like to admit.

Perhaps the best approach is to perform one's own
experiments with one's own car.

 

"L Alpert" <alpertl@xxgmail.com> wrote
> Good real world application.

Ditto.

What Gordon found appears to be consistent with some study
(or a summary of a study) I read not too long ago. Tom and
Ray of "Car Talk" may have cited it at their web site, if
memory serves. Namely, even when moving at highway speeds,
not using the A/C and leaving windows open for some cooling
is best.

Plenty of studies on the net on this. E.g.
---
Several sources claim that closing windows and using a car's
air conditioner will provide better fuel economy at freeway
speeds than leaving the windows open without the air
conditioner. However, FSEC tests showed this in not the
case. In repeated evaluation at 65 miles per hour, our test
car experienced 11% better fuel efficiency with no A/C and
the windows open than using the air conditioner. We also
found that closing windows at freeway speeds improved fuel
efficiency by 2-3%.

http://www.fsec.ucf.edu/Pubs/energynotes/en-19.htm
---

The above site has a nice chart for freeway speed driving.
The test vehicle was a 1986 VW GTI.

OTOH, the damned people at FSEC.UCF appear to misquote a
study done by edmunds.com, indicating that there's no
difference at highway speeds.

I suspect this all depends on the model and its wind
resistance characteristics more than we'd like to admit.

Perhaps the best approach is to perform one's own
experiments with one's own car.

 

"L Alpert" <alpertl@xxgmail.com> wrote
> Good real world application.

Ditto.

What Gordon found appears to be consistent with some study
(or a summary of a study) I read not too long ago. Tom and
Ray of "Car Talk" may have cited it at their web site, if
memory serves. Namely, even when moving at highway speeds,
not using the A/C and leaving windows open for some cooling
is best.

Plenty of studies on the net on this. E.g.
---
Several sources claim that closing windows and using a car's
air conditioner will provide better fuel economy at freeway
speeds than leaving the windows open without the air
conditioner. However, FSEC tests showed this in not the
case. In repeated evaluation at 65 miles per hour, our test
car experienced 11% better fuel efficiency with no A/C and
the windows open than using the air conditioner. We also
found that closing windows at freeway speeds improved fuel
efficiency by 2-3%.

http://www.fsec.ucf.edu/Pubs/energynotes/en-19.htm
---

The above site has a nice chart for freeway speed driving.
The test vehicle was a 1986 VW GTI.

OTOH, the damned people at FSEC.UCF appear to misquote a
study done by edmunds.com, indicating that there's no
difference at highway speeds.

I suspect this all depends on the model and its wind
resistance characteristics more than we'd like to admit.

Perhaps the best approach is to perform one's own
experiments with one's own car.

 

"L Alpert" <alpertl@xxgmail.com> wrote in message
news:AIVMg.5$7C7.2@newssvr12.news.prodigy.com...
>
>
> The net effect will be determined by environmental conditions and choice.
> My preferences are usually determined by speed of travel and
> temperature/humidity conditions. <90° and lower humidity while driving on
> back roads at <50 mph would be open windows for me for the most part
> (general rule of thumb). >90° would most likely be AC all the time.
>
> Highway speeds would either be AC or vent, depending on the preferred
> comfort level at that specific time. I am not very concerned about the
> differences in fuel economy, only in comfort level (although it is always
> nice to know.....).
>

That's undoubtedly the bottom line. I used to live in Phoenix, and the
windows don't open wide enough to make 117 F comfortable!

Mike

 

"L Alpert" <alpertl@xxgmail.com> wrote in message
news:AIVMg.5$7C7.2@newssvr12.news.prodigy.com...
>
>
> The net effect will be determined by environmental conditions and choice.
> My preferences are usually determined by speed of travel and
> temperature/humidity conditions. <90° and lower humidity while driving on
> back roads at <50 mph would be open windows for me for the most part
> (general rule of thumb). >90° would most likely be AC all the time.
>
> Highway speeds would either be AC or vent, depending on the preferred
> comfort level at that specific time. I am not very concerned about the
> differences in fuel economy, only in comfort level (although it is always
> nice to know.....).
>

That's undoubtedly the bottom line. I used to live in Phoenix, and the
windows don't open wide enough to make 117 F comfortable!

Mike

 

"L Alpert" <alpertl@xxgmail.com> wrote in message
news:AIVMg.5$7C7.2@newssvr12.news.prodigy.com...
>
>
> The net effect will be determined by environmental conditions and choice.
> My preferences are usually determined by speed of travel and
> temperature/humidity conditions. <90° and lower humidity while driving on
> back roads at <50 mph would be open windows for me for the most part
> (general rule of thumb). >90° would most likely be AC all the time.
>
> Highway speeds would either be AC or vent, depending on the preferred
> comfort level at that specific time. I am not very concerned about the
> differences in fuel economy, only in comfort level (although it is always
> nice to know.....).
>

That's undoubtedly the bottom line. I used to live in Phoenix, and the
windows don't open wide enough to make 117 F comfortable!

Mike

 

"L Alpert" <alpertl@xxgmail.com> wrote in message
news:quUMg.17456$1f6.9243@newssvr27.news.prodigy.n et...
> Gordon McGrew wrote:
>> On Sat, 9 Sep 2006 16:41:34 -0700, "Michael Pardee"
>> <michaeltnull@cybertrails.com> wrote:
>>
>>> Vehicles with lower fuel economy are less sensitive to differences,
>>> because the losses are already pretty high. In the Toyota Prius fora
>>> people are shocked to find that running the heater can decrease the
>>> in-town fuel economy 10 mpg... the finer the edge the more quickly
>>> it dulls.
>>
>> Running the heater? Isn't this just waste heat anyway?
>
> I guess it has to do with the amount of juice needed to run the
> fan.......????
>
The effect on the amount of time the engine runs can be pretty radical. The
car has a display for the mpg over 5 minute intervals, so the first winter
we had it I decided to see just how much the heater would drag it down. I
opened the windows and turned the heater on full while driving in town. The
previous five minute bars had been something like 40 or 45, while with the
heater on full it dropped to 25! Even at 75 mph with the A/C on full and
five adults in the car it never went below 30 in real life.

Mike

 

"L Alpert" <alpertl@xxgmail.com> wrote in message
news:quUMg.17456$1f6.9243@newssvr27.news.prodigy.n et...
> Gordon McGrew wrote:
>> On Sat, 9 Sep 2006 16:41:34 -0700, "Michael Pardee"
>> <michaeltnull@cybertrails.com> wrote:
>>
>>> Vehicles with lower fuel economy are less sensitive to differences,
>>> because the losses are already pretty high. In the Toyota Prius fora
>>> people are shocked to find that running the heater can decrease the
>>> in-town fuel economy 10 mpg... the finer the edge the more quickly
>>> it dulls.
>>
>> Running the heater? Isn't this just waste heat anyway?
>
> I guess it has to do with the amount of juice needed to run the
> fan.......????
>
The effect on the amount of time the engine runs can be pretty radical. The
car has a display for the mpg over 5 minute intervals, so the first winter
we had it I decided to see just how much the heater would drag it down. I
opened the windows and turned the heater on full while driving in town. The
previous five minute bars had been something like 40 or 45, while with the
heater on full it dropped to 25! Even at 75 mph with the A/C on full and
five adults in the car it never went below 30 in real life.

Mike