More of a curiosity question, but I'm trying to figure out the most fuel efficient way to run the AC in my car, assuming I must use AC. I already know when to open windows vs use AC, so I'm looking at what to do if I'm traveling fast enough that AC is the better option.

I have three options I can set. I can increase the fan, I can increase the 'cold' value for the AC, and I can turn on and off the AC. If I'm trying to figure out a way to keep the temperature cool enough to be tolerable which of these options do I use?

I would assume that usually having the fan on max value with the actual 'temperature' set a lower setting (cooler, but not cold), and leaving that on all the time, would be the most efficient. However, that would depend on exactly how the AC works and how smart it is about adjusting the power of the AC when I keep the temperature on a warmer setting.

Anyone have a more definitive answer?

  • Please see my Answer below with the experiment of painting the roof of a wagon white. Incredibly helpful. Go for a car that is white, silver, beige, etc. OR, paint the roof if it is old.
    – user2423
    Jul 30, 2015 at 12:44
  • 1
    @nocomprende it was white when I got it ;)
    – dsollen
    Jul 30, 2015 at 14:28
  • 2
    Whatever the answers you get, don't forget two things: 1. (Assuming your car is in the sun before driving) Cool the car as far as possible before switching the airco on (all doors open two minutes before driving, all windows open when you start driving, one minute later the ventilation full on so that the motor compartment can get rid of the accumulated heat). And 2. Turn the AC off 10 minutes before you reach your destination.
    – user2451
    Aug 11, 2015 at 10:34

5 Answers 5


If you have a "climate control" system where you can set a temperature and let it do it's thing, it's probably best to let it do it's thing on auto ("auto eco" if it has such a setting).

If you have more manual controls, then I'm guessing that the compressor uses the same amount of power regardless of how the other controls are set. On the basis that the compressor dominates the power consumption, then, I would suggest,

  • Keep the temperature set as low as is comfortable to you (angle vents so that the air is not actually hitting you, to avoid uncomfortable chill)
  • Run the fan at a moderate speed (too fast and the air doesn't have time to be cooled much. And it's noisy, and itself burns power)
  • When the cabin has cooled enough, leave the other settings as they are and turn the A/C off. When it gets too warm, turn the A/C on again.

I don't have any evidence or specialist knowledge, but I'm guessing that that will be more energy-efficient than adjusting the temperature control for a steady continuous level of cooling.

  • 3
    Every compressor I've seen is either on or off, so it uses the same amount of energy regardless of your control in the vehicle. Your temperature control handles the balance between the a/c coils and the heater core. Also, be sure to set recirculation on so you are cooling the already cool air in the cabin, not pulling in hot air from outside.
    – rpmerf
    Jul 15, 2015 at 16:25
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    @rpmerf: You should add recirculation point as an answer. Jul 16, 2015 at 6:24
  • I think that the compressor cycles on and off based on the cooling load to some degree, at least my car does. The temp control formerly (not sure if it still does) only brought coolant from the engine in to the heater core to warn the air or alternately opened a louver to allow air through the heater core (because water always circulated in it), so when it is not cold the temp control is worse than useless.
    – user2423
    Jul 28, 2015 at 23:21

The existing answer by Simon W is excellent, I just wanted to add some thoughts to it.

  1. The best thing is to have your car running as efficiently as possible overall, so make sure the engine is cared for. Car engines have a theoretical max efficiency of less than 50% (based on thermodynamics) so this is an absolute limit no matter how we re-design cars. Actually driving anywhere wastes even more (wind resistance).
  2. I don't think that a good A/C system adds all that much to the engine's workload. I have a 19 year-old Subaru wagon (not the most efficient or newest thing) and it gets about 30 MPG on the highway, with or without A/C on. You could do worse with a brand new car! In the city with long stops at lights, it gets about 21, maybe 24 without the A/C, but the sitting is really what wastes fuel! ZERO MPG! Find alternate routes around those incredibly long stoplights.
  3. When I do have to sit at a light, I put the transmission in Neutral. This makes it "lug" less, and does not heat up the transmission. Also the A/C works better with the engine idling freely. Just remember to put it back in gear: people are not impressed when you go Vroom! and just sit there, after waiting two minutes at a stoplight.
  4. I have tried to cut the incoming heat-load on this large, dark brown car in a hot sunny climate. I leave the windows open a bit when parked during the day, when possible, to lower the temp as much as I can when I first get in. I have painted about 15 sq ft (most of the roof area) white - just tried that so I am not sure how much it helps. I draped white paper over the roll-up cover in the rear area to reflect heat away. I park in the shade (this is the most important thing, but not always in your control).
  5. I am trying to find a simple solar fan solution to vent the heat while the car is parked without letting in rain or blown debris.
  6. The A/C system has a slow leak, so I top it off with a can of refrigerant, maybe once a year. Keeping it full up makes it work much better.

Those are my suggestions. Painting the roof is a bit radical, but Lursa is old and has basically no resale value, and is mine. The roof is not too visible unless you are tall enough to look over the car.

  • 1
    Re: painting the roof white - the results are in! I put my hand on the white part: comfortably warm. Brown part one foot away: scorching, could not leave hand there! If one sq ft has about 100 Watts equivalent of inSOLation (not insulation) then I have reduced the heat load by one plug-in room heater - 1500 Watts. Probably cuts the total heat in to the car by HALF. Wildly effective. Too bad I could not use mirror finish, but I would blind truckers.
    – user2423
    Jul 30, 2015 at 12:38
  • To add to these excellent suggestions, I have a windscreen protector with silver fabric on one side. It stretches all the way acrosss and has flaps you can trap in the doors to keep it in place, with elastic you can hook together inside to help. This thing is amazing, and was very cheap at the hardware store. Also very useful in winter.
    – RedSonja
    Aug 1 at 12:37

I drive a 2015 Corolla. The most efficient way I figured to keep the car cool was to turn off the AC, turn the temperature to coolest, and just have the fan on. This setting gives enough cold air. Even with the AC off, we sometimes have to turn the vents away from us to not get too cold. It does takes a little longer to cool the car when starting, but then, you can turn the fan to the highest speed until the car cools down to a comfortable temperature. It has worked for me for temperatures till low hundreds.

A humble request to everybody, there is no need to race to a stop sign, or a light turning yellow, let the car coast to it. Neither do we need a sudden acceleration while start on a green signal. I don't want to put the CO2 in air that I have done nothing to take out.

Lower gas consumption from all of us will can result in lower gas prices, and I never heard that lower gas prices can hurt the economy of the nation.

  • 1
    In areas with low humidity (< 35% or so) yes, air blowing on you can be sufficient. But if you are driving to work, you might have more clothing on, and if it is humid, no amount of steam blowing on you will do much good, because you have to sweat (even unnoticeably) to avoid heat stroke. If you have the HVAC selector pointed at "defrost / defog" (windshield icon) then the A/C will be ON even if you didn't push an A/C button. If the air is mysteriously cool, that is not magic.
    – user2423
    Aug 11, 2015 at 21:28
  • @nocomprende - It is definitely not magic :). I am just trying to say that we can do without turning AC ON, of course except for the humid conditions as you said. My car's MPG falls down by at least 2 MPG when the AC is kept on. I live in mid CA which is good about humidity (so far), so I don't always have to turn on the AC. Just the fan on coolest works fine. Aug 19, 2015 at 17:11

Assuming the car has been in hot sunlight but the outdoor temperature is tolerable, the most fuel efficient way is to ventilate without any AC, just by using fan at max speed, blend door at coldest possible position and keeping the windows open. Once the temperature of air inside the car becomes tolerable, then turn on the AC, set the fan to low speed, set blend door to coldest possible position and close windows. If you hit high speeds (80 km/h and above) before the temperature of air inside the car becomes tolerable, then close windows when hitting that speed (earlier than you would otherwise close them), turn on the AC and set fan to a speed that cools the car interior at a comfortable pace. If it becomes too cold, you may slightly increase the blend door to warmer, but the preference is this: first reduce fan speed, and only then if it doesn't help set blend door to warmer position.

The reason for these is that the performance of keeping windows open is best with largest temperature difference (outdoor air comfortable, air inside car very hot). The fuel efficiency of keeping windows open exceeds that of air conditioning if you are traveling at slow speeds. Also if you cool down air and then use blend door to mix in air heated by engine, that's just wasting energy. Therefore the blend door should be always at coldest possible position.

In theory, if the outdoor air isn't very hot, and you drive the entire trip at slow speeds, in that case you could of course keep the windows open during the entire journey. But the savings would be small, and it's hard to control the temperature precisely, so even minor priority on comfort dictates using the AC instead of keeping windows open.

In automatic climate control systems, the automatic mode works as follows:

  • AC is on
  • Initially, fan is at very high speed, blend door at coldest position
  • Then, when temperature drops to near the setpoint, fan will be at low speed, blend door at coldest position (but if it nevertheless becomes too cold, then the blend door will be moved to mix some air heated by the engine)

This is the best plan if you're not going to open the windows at all. However, it wastes a bit of fuel compared to initially rapidly ventilating with all windows open when driving at low speed, and only after that using the automatic mode of the climate control system.

If outdoor air is very hot, then windows should never be opened. Instead, just use the AC.

Most recent cars have an AC compressor that has variable power. Thus, to save on the energy, you want to reduce the load on it as much as you can. You don't do that by cooling down lots of air and then mixing hot engine air using the blend door -- instead, you move little air with blend door in coldest position.


Use the ac when going downhill. Use regular fan on low temp setting uphill. Don't set the temperature really low, maybe 22*C means working the compressor less and helps us acclimate to it being summer when we get out.

  • Can you explain how running the AC while going downhill is better from a sustainability sense? Seems to me the compressor will draw a similar amount of power in either scenario (and therefore burn a similar amount of gas).
    – Robotnik
    Jul 14 at 4:40

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