How much more (roughly) gasoline my car uses when I drive with my air-con on.
5 % , 15 %, 20 % ?
It’s 1.6 Nissan ‘Sentra’ sedan
Windows down vs. air conditioning from Myth Busters:
“Urban puzzle”: it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car’s drag).
Computer-based mpg measurements:
* 11.7/11.8 with A/C on and windows up
* 11.7/11.8 with A/C off and windows up
* 11.3 with A/C off and windows down
So, according to the computer, it’s better to use A/C with windows up.
This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you’re-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).
Jamie’s A/C car ran out of gas first – Adam’s windows down SUV ran for another 30 laps – completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.
My comment: Windows down and no A/C = That’s something like 20-30% better mileage. I forget the length of the track and the EPA est MPG of the vehicles, but 30 more laps when you start out with just 5 gallons of gas is a significant difference?! I got a mental note that it was 20-30% better mileage.
And that is what invalidated it. The whole point is that *at high speeds, *windows open generates lots of wind resistance. And, to me at least- it seems to increase dramatically around 55-60MPH. In other words, I can drive 45 windows down and it is nice, but 65 and the wind and noise is nasty.
In any case, the difference is small and is less than your driving style.
The answer is “it depends”. With the windows open, the drag on the car is increased, but the drag depends on how fast you are moving also. So, if you are moving very slow (like in city traffic) the drag may be almost negligible. At highway speeds the drag is extremely significant.
The air conditioning system on the other hand provides a constant load on the engine. How much of a load depends on how big the air conditioning system’s compressor is compared to how big the engine is. A tiny four cylinder engine in a car with a relatively large passenger compartment is going to be more loaded (percentage-wise) than a car with a large engine and a smaller passenger compartment.
At a dead standstill, it’s always more efficient to roll down the windows. At highway speeds, it is always more efficient to have the windows up and the AC on. For every car there’s a break even point somewhere in the middle, usually at a relatively low speed, which I’m gonna guess for most cars is somewhere in the 20 to 30 mph range.
The most efficient way of course is windows up and AC off. Depending on how hot it is outside you might pass out from heat stroke if you do this, though.
The only real numbers I can give you are for my own car. I have a computer on my car which calculates gas mileage and such fairly accurately. The difference between going to work with the AC on and the AC off (windows up in both cases because the weather would allow it) was down in the tenths of a mpg. So, for my car at least, I get more of a difference between driving in summer vs. winter than I do for AC on vs. AC off. My mileage drops by about 1 mpg in the winter, although I’m not sure if this is because of the cold air or the fact that they put ethanol into the fuel here in the winter time.
My car is a large Caddilac that only gets 21 mpg anyway. My drive to work is at 40 to 50 mph on a two lane road with few stop lights. YMMV. Literally.
Cecil addressed this back in 1986, though he concluded by saying, “Obviously more research is called for. Unfortunately, by the time we get around to it, we’ll probably all by using air cars powered by nuclear reactors.”
You may also be interested in the Staff Report Does using your car radio reduce your gas mileage?
From personal measurements over the past 25 years with 4 cars (4 door sedans, 4 and 6 cylinder engines) and 2 small (4 cyl.) pickups, the A/C costs me from 8 to 12% in gas mileage. This is mostly with windows up. In my corner of the world (North Texas), there aren’t really enough “windows down” days to get a good reading on it. And virtually no one drives with the windows down in the summer here unless they have no choice.
The numbers I heard about 25 years back were: Below 40 mph, windows down is cheaper. Above 40 mph, windows up and A/C on is cheaper. However, that is just a general rule of thumb. There are a lot of variables that will affect your particular situation such as the already mentioned relative sizes of the A/C compressor and the engine, but also the temperature of the outside air and the cabin temperature. As these temperatures rise, so does the power consumed by your A/C.
In the mid 80’s, a tractor of ours was required to be dyno tested. Easy for a tractor, just use the PTO. After seeing the engine was in spec (90hp), I got the dyno operater’s permission to see what happened with the A/C on. We only ran it for a moment, but output dropped to 85 hp. I’ve been told the A/C compressors in tractors are a lot bigger that what’s found in cars etc. When ringing around for a replacement compressor for my MF 3095 a couple of years ago, I was told the same unit was used in bus’s (What is the plural for bus anyway? :smack: ).
My 3095 has a fuel flow meter, I think the difference between A/C compressor on/off at idle is 0.3 litres per hour at idle. However the duty cycle seems quite low, so the actual fuel use is lower than 0.3 l/h
Thanks guys , your answers were very informative … as usual.
A factor not yet mentioned is engine displacement.
A large engine operating at light load (for it) is pretty inefficient, mostly due to pumping losses. Adding a bit more load increases the power requirement, but the power (not milage) efficiency actually increases. To a large extent, this compensates for the increased load. This is true up to about 70% of maximum torque, after that efficiency starts to drop, partly due to reduced fuel vaporization, but also due to (reasonable) tuning choices made by the designers.
If a small engine is operating at ~70% torque or greater, just to maintain speed, adding the additional load of the AC pushes the engine into a LESS efficient operating area, so there is no improved effiency to partially compensate for the increased load, thus the effect on milage will be much greater…note that the econobox will still be getting better milage than the SUV though.
Note that the above applies only to spark ignition engines. Diesels are a whole different ball game.