Air Conditioning vs Gas Mileage

Adelard of Bath · Post by **Adelard of Bath** » Mon Jan 17, 2005 11:24 am

Since it is currently about one degree Fahrenheit outside my house right now, this seems like the perfect time to remind everyone, that if you are cruising down the highway, and it is WARM out, and you think to yourself "I want some air" so you roll down the windows...STOP! cuz open windows create serious drag and waste far more energy than running your air conditioner. Just a little somethin-somethin that I like to fluster the hippies with...

It may be counter intuitive but think about all that noise with the windows down, and the wind whipping around...major drag...around town it shouldn't be a problem but we are talking "road trip" "highway cruising" when gas mileage really matters

I remember back in college I was in a "Fluid Dynamics" class and we did some calculations...both for open windows, and then for just an open sunroof, I think we used a mid-90s Saab for the example, or something like that....it was SHOCKING to see how much open holes (windows etc) in the solid car changed the drag. Whereas the a/c uses....a small amount? Especially if you run ours in that cool little "econ" mode which just cycles the compressor probably half as much. But I find the a/c to be often too cold anyway.

Those of you without a/c....just think of the gas you are wasting!

takza · Post by **takza** » Tue Jan 18, 2005 9:31 am

Best mileage I ever got was with the AC on...once you turn it on...there's no point in trying to go fast anyway...so I cruised real easy...in ECON mode.

If you like the windows open and also to have little air drag...try the wind deflectors they sell for the side windows...they stop all the noise and wind buffeting...but you also don't have a lot of air blowing in.

Problem is that the only one's I've found for the Tercel are made of stainless rather than acrylic.

Post by **Mattel** » Tue Jan 18, 2005 6:53 pm

I drove up to Queensland and back from Melbourne 2 summers ago after just getting the A/C regassed

it was 37-40 degrees outside and we ran the A/C on econ after 10minutes on normal. Over a return distance of over 3,500 kms with 90% freeway driving at 100kph we averaged 8.5 litres per 100km with A/C on non stop

I had the tyres at about 36 psi what do you guys drive at for road trips?

takza · Post by **takza** » Wed Jan 19, 2005 8:10 am

I calculate 27.6 MPG? I can't think in metric.....I can barely think in Olde English......

As far as tires...depends on the tire...current Kumhos I run at top pressure of 44 PSI. Best overall MPG has been around 33 with heated fuel. Highway at 35.

Old cheapo tires I ran at 32 PSI out of 35 max....these ran pretty hard at this pressure. Got 35-36 MPG overall....no heated fuel.

2wagons · Post by **2wagons** » Wed Jan 19, 2005 4:31 pm

In my 83 I got 30-35 mpg at altitude along the Front Range of Colorado-dear old Colorado, sniff, at 60-70 mph. Now in Virginia, with the 87, I get around 27-29 mpg, might get more with new plugs and etc. I think the lower cruising RPM of the 83 really made a difference in the MPG.

Retroloc1 · Post by **Retroloc1** » Thu Jan 20, 2005 10:43 am

2wagons wrote: In my 83 I got 30-35 mpg at altitude along the Front Range of Colorado-dear old Colorado, sniff, at 60-70 mph. Now in Virginia, with the 87, I get around 27-29 mpg, might get more with new plugs and etc. I think the lower cruising RPM of the 83 really made a difference in the MPG.

At top speed, a 83 with it's tall axle ratio turns 400 to 500RPM lower than a 87.

Ogedei · Post by **Ogedei** » Fri Jul 15, 2005 12:02 pm

Watched a mythbusters last weekened that took this on.

When attached to the computer the SUV's proved the point that the AC was less fuel efficient.

However if you watch Mythbusters you know they go out of the way to prove it right or wrong either way.

So they drove identical SUVs at 55 mph around a track with equal gas

One with Ac on max one with windows down.

The windows down went 30 miles further. Seems that perfect conditions vs. real life has some discrepancies.

I have considered a test of my own on this but do not know when I can clear time.

3A-C Power · Post by **3A-C Power** » Fri Jul 15, 2005 12:25 pm

Depends on the individual conditions of the vehicle. Like how efficient the AC is and how much drag there is overall. My guess is on the SUV the overall drag is so high because of the frontal area that the windows add proportionately less drag than they would on a smaller car. It would also depend on the efficiency of the AC system especially if you're using economy mode which cycles it on and off. R12 systems are also more efficient than R134a, or so I've heard.

Too bad the AC is so much less likely to work than the window winding mechanisms. After reading this, I want my AC to work again.

Mac · Post by **Mac** » Sun Jul 17, 2005 11:27 am

I wouldn't be supprised if mythbusters overfilled the gas tank in the SUV with the windows down, and also, who knows how one SUV was maintained compared to the other? the only accurate way would be to get two brand new vehichles, somehow fill them both with the exact same amount of fuel (your gas gauge is too inacurate to know for sure) and then test which method works better.

the problem is with the mythbusters guys is that they aren't really that smart, and they tend to overlook alot of things.

on my echo i have AC and it makes a tiny differance on performance, but the modern systems vary themselves depending on cabin fan speed, outside tempature, engine RPM, so the only time it will use alot of gas is when its really hot outside and the engine is at cruising speed, which is when i won't notice a performance hit.
when i first got the car and i was driving super nice during the break in period, I got 850KM to a tank (40-something MPG) city. i found out after that was with my AC on because i always leave my fan selector on defog so its blowing air on the windshield and i didn;t realize that automatically turns on the AC when its set it to defog.