I know that having it on uses more gas, but does having it on a high setting rather than low use more gas?
Thanks in advance.
Oops. Someone please ask a mod to move this, thanks again.
Yes. The cooler you want your interior kept, the more energy (and hence gas) it requires. TANSTAAFL and all that.
While Q.E.D. is correct I believe that the amount of additional fuel used between high setting and the low setting is inside the accuracy of how you are measuring your fuel mileage.
You measure fuel mileage using your odometer (probably accurate to between +/- 2% and +/-10%) and your gas gauge which is maybe accurate to 1/2 gallon. In other words both of the items used to calculate fuel mileage are not what anyone would consider to be precision measuring devices.
I believe that the difference is so small and your measurement devices so crude that you will not notice a difference out in the real world.
I though it would use more. The last time my mechanic worked on my car, I got it back with the A/C stuck on high. I asked him to fix it and he did, but he told me it uses the same amount of gas whether it’s on high or low. Not that I’d even want it on high all the time, anyway, but I didn’t think he was right on that. Thanks. Nice to be able to come and ask a simple question and get quick and accurate replies.
I would imagine it to be miniscule. All we are talking about is fan speed here right? The compressor isn’t working any harder.
So you will be pulling more electrical energy from the alternator, but I can’t imagine that it would matter hardly at all.
The compressor has to to work longer on high than on low. The colder it is inside the car the more heat enters from outside. The compressor has to put that heat back outside and hence must run for a higher fraction of the time on high than on low.
Alice the Goon’s mechanic is just plain wrong although as Rick said, it might not be noticeable.
So the compressor cycles on and off? I thought that once the AC is kicked on, the clutch for the compressor kicks in and it just runs. The amount of cool would then be controled by fan speed.
But on every car I’ve ever seen the thermostat and fan speed are separate controls, and when I turn the thermostat down I do not hear any change in the fan speed.
I thought it was some sort of valve that controlled a mixture of cooled vs. uncooled air somehow.
But if you run the compressor the same amount and kick the fan speed up, you’d just be blowing in more, but less cool, air. Simply increasing the fan speed won’t increase the amount of air that gets cooled.
Anyway, the answer to the OP is “of course.” Using ANY accessory in your car - AC, radio, power windows, whatever - draws more power away from the engine because the power doesn’t just come from nowhere.
In the case of the AC it’s probably slightly measurable over long distances. I think people are underestimating the power needed to run the AC; if you idle your car and turn it on you can hear the engine compensating for the draw. I have had mechanics tell me the AC by itself can take 4 to 15 horsepower from the engine to run, depending on the vehicle - a smaller vehicle has smaller Ac components - that’s not a lot in a modern engine, but it’s measurable. If you controlled your measurements well enough, I bet if you took a 500-mile wide-open freeway trip with the AC off and then turned around and took the same trip with it on you’d notice a difference in gas consumption. AC takes a lot of juice. In the case of most any other common accessory it’s too trivial to notice.
Here is a paper on the subject. PDF warning!
Wait you are both right. I was less than rigorous in my answer last night. It depends on the system.
On some cars the compressor cycles on and off to maintain temperature. These systems are referred to a Cycling Clutch Orfice Tube systems (CCOT) Tuning the AC to max would mean that the compressor runs longer between cycles. (greater duty cycle)
However other systems use an expansion valve that opens and closes to allow more or less refrigerant into the evaporator in response changes in conditions. The compressor does not cycle in this type of system.
So if you have a CCOT system on high compared to low, you will have the compressor running at a higher duty cycle (on a greater percentage of the time) and you will consume more power to run the fan at higher speed.
If on the other hand you have an expansion valve system the compressor is already running, by putting the fan on high you car causing the valve to open and admit more refrigerant into the evaporator. Of course you would still have the increased current consumption of the fan.
These are the two main types of systems. There are a bunch of variations on these two main types.
The difference in fan speed might be somewhere around 70 watts on low, and 300 watts on high. I will leave the math as to how much gasoline it takes to run that extra 230 watts to someone else. 
Rick Jay No one here is arguing that using AC does not use more fuel, that is not the question. The question is if you already have the AC on low what effect is there on fuel mileage if you turn it to high? Also is that effect measurable. As I noted in my last post, on some systems the compressor is already running, and you just have the increased load of the fan to contend with.
These answers contradict my understanding of how most modern car A/Cs work. Usually, the compressor cycles independently of the temp setting and the air temp is controlled by a valve that mixes in outside warm air with the cooled air.
Edit: Whoops! I see that I should have refreshed the page before replying. Please ignore.
Didn’t Mythbusters confirm that the AC does cost you some mileage? [searching Wikipedia]
Strange, I know they did it, but can’t find the episode. Or perhaps I’m delusional…
In either case the compressor has to do more work. In the expansion valve controlled system, when you turn the A/C to high more refigerant is circulated. In other words the compressor compresses more refrigerant in a given time and thus does more work.
Not exactly. What they tested was the difference in fuel economy between using the AC vs. rolling your windows down.
Episode 22. They took two nearly identical trucks and filled them with the same amount of gas. One they drove with the windows down, the other with the A/C up. They drove until they ran out of gas, and the windows down truck made it 30 laps further.
Nevermind, I finally found it-season 3, episode was “Mythbusters Revisited”, but they tested a car with the AC running vs. a car with the windows down, a slightly different thing than what’s in the OP.
Agreed, but still the difference is inside the accuracy of the instruments that the car owner has at their disposal to measure.
It’s like trying to measure a piston to .001" with a yard stick that is only accurate to a quarter of an inch.
my post was just to emphasize the point that running the A/C harder uses more gasoline, for those still unsure. And that is, of course, true whether or not you can measure it on your fuel gage.
Slight hijack: I have heard that at least some “High” AC settings simply close the vents. On High AC, the car continually cools the same air, whereas on low it is constantly sucking in more air to cool. Does this exist or was I misinformed?