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AC vs windows open mileage test

Old 08-22-2007, 03:13 PM
bwilson4web's Avatar
Engineering first
Join Date: Oct 2005
Location: Huntsville, AL
Posts: 5,613
Default Re: AC vs windows open mileage test


My wife and I just drove from Huntsville to Nashville and back at 65-70 mph with AC. The trip mileage came in just about 49-50 MPG with the AC (I wasn't working on accuracy, just trying to get her to an appointment.) At ordinary cruising speeds of 65 mph, I would expect about 53-54 MPG. But consider this, the EPA highway rating (old style,) was only 45 MPG and they used a slower average speed, 48 mph, without AC.

AC isn't free but it sure is affordable . . . and makes the wife much happier.

Bob Wilson
Old 08-23-2007, 01:07 PM
Join Date: Jul 2007
Posts: 27
Default Re: AC vs windows open mileage test

Originally Posted by KLCarch View Post
Ever since I saw the 'mythbusters' guys do an AC vs windows test I've wanted to do one with my hybrid. They ran 2 expeditions around a track at 45mph: one with all the windows wide open, and one with AC and closed windows. the windows open car got better mileage. But, they weren't travelling at real highway speeds, so I've been wondering how much open windows really hurt.

So this past saturday I had the oportunity to do a highway run where it was fairly flat (unlike where I live). I had to pick my kids up in Corning, NY. The highway there- Rt 17 runs for 50 miles between Binghamton & Elmira pretty flat. there are some slight grade ups & downs, but not much.
I ran 3 rounds of each venting option- driving at mainly 60mph- but varying slightly between 55-64mph with the grade. I figure 3 rounds each way would average out grade & wind variations. roads were dry, temp was 85. rt 17 is a divided 4 lane in this area (2 each way). speed limit was 65; I was passed, but traffic was light, and mostly going the limit, not above. going west, I had a suburban about 10 car lengths in front of me as a windbreak. he was travelling at 60mph.
3 venting options: AC w/ windows closed; no AC- windows closed; no AC diagonal windows- 1 each side of car- open 6"

Driving West - 2 adult passengers:
AC no windows: 56 52.6 56.7 avg: 55.1

no AC/ no windows: (hot!) 64.7 62.8 59.4 avg: 62.3

no AC/ 2 windows venting 65.1 55.6 55.6 avg: 58.7

Driving East - 2 adults, 2 kids:
AC/ no windows: 54.1 54.1 52 avg: 53.4

no AC/ no windows: 63.2 55.6 57.9 avg: 58.9

no AC/ 2 windows venting: 57.1 60.0 58.6 avg: 58.5

overall mpg averages:

AC/ windows closed: 54.25mpg
no AC/ windows closed: 60.6mpg
no AC/ 2 windows venting 6": 58.6mpg

thus confirming the mythbusting theory to me: AC takes alot of energy & is less efficient- at least with these small engined hybrids. I did these test on flat terrain, but I know with my usual driving over hilly roads, the car really works to go up a hill and run AC. I don't use AC much myself anyway- but I'm in the northeast & it doesn't get over 90 much here.
perhaps 40 years ago, when test were first run for fuel efficiency with AC, the bigger powerplants were more efficient with the windows shut and AC, as well as larger window opening volumes and a generally less aerodynamic form overall.

on a side note- this is my first full tank since the recall- I had my oil changed to mobil 1 as well & I got my best tank ever: 55mpg. I love this car- it just keeps getting better & better. I DREAD the return of really cold weather this winter!

Doing a T-Test on your data shows that "AC no windows" average MPG is statistically different from the average MPG of the other two methods. However the averages for "no AC windows closed" and for "no AC 2 windows venting" are not statistically different.

Granted though, you do not have a lot of data points here.
Old 08-23-2007, 02:25 PM
Join Date: May 2007
Location: McKinney, TX
Posts: 22
Default Re: AC vs windows open mileage test

Very interesting data and testing.
Any one know if opening windows parallel would create much more drag? I'm asking, as a driver of a coupe, and would only be able to vent the front 2 windows.
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