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?)

I checked the calculated fuel economy over approximate ten mile
segments on each tank of fuel. I used the cruise control and the
calibrated Scan Gauge mph measurements. Usually I could go the whole
ten miles without touching the gas or brake. I tried to be as
consistent as possible.

For open windows, I started out rolling them all the way down but
after a couple segments of that, I decided that no one could stand the
tornado effect for long distances so I tried various partial open
positions which improved ventilation without being punishing.
Generally this was the rears open 4 inches and the fronts either
closed or open 3 inches.

The vehicle, unfortunately, was not typical for most drivers: 1998
Odyssey 4 cylinder with a Thule car-top cargo box. Newer A/C systems
and less drag-challenged vehicles may yield different results, but
here goes...


Test 1 8/31/06 76 mph I35 Southbound Hilly
22.8 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 4 19.83 100
Off Full Open 2 19.45 98
Off Part Open 2 18.90 95
On Closed 4 18.24 92


Test 2 9/4/06 75 mph I35 Northbound Hilly
19.7 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 5 21.62 100
Off Part Open 3 21.27 98
On Closed 1 20.90 97


Test 3 9/5/06 75 mph I35 Northbound Hilly
23.4 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 3 20.43 100
Off Part Open 2 20.65 101
On Closed 3 19.27 94


Test 4 9/5/06 73 mph I55 Northbound Flat
21.6 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 6 22.22 100
Off Part Open 5 21.68 98
On Closed 5 20.82 94


Overall, weighted by number of segments relative to windows up, A/C
off, fuel efficiency was:

Windows open 98%
A/C on 94%

In this test, turning on the A/C cost three times as much fuel as
opening the windows. There was no indication that it made any
difference whether the windows were wide open, rears down 4 inches or
front and rear both down 3 - 4 inches, but there was limited testing
of this, and the effect was small in any event.

As noted before the vehicle may not be typical, but these were the
results. One further point of interest; shortly after I started one
segment I came upon a speed reduced work zone. I didn't use the
segment in the above calculations but I noted that average speed was
63 mph and average fuel consumption was 26.4 mpg. It appears that the
difference between going 60 and going 75 was about 4 mpg.