Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Most efficient use of AC

There are times when the AC is too cold. Is it more efficient to turn it off for a while and then turn it on again when the air is too warm, or is it more efficient to change the heater setting from max cool to something warmer? If I do the latter, won’t the AC be working at max while the controls add heat to the air coming out, and so the engine works harder? I have a 2005 Toyota Camry, but my question probably applies to all makes and models.

If you blend to regulate temperature, the A/C compressor runs all the time. If you manually cycle the A/C, then it only runs when you ask it to…

On most cars, with the fan on low and the A/C set to “normal” (outside air) it’s pretty hard to get too cold…

If the A/C compressor is running, the “load” on the engine is the same regardless of where you adjust the other settings…

It is more efficient to turn it on and off as needed, but that is also annoying after awhile. If you live in hill country, you can maximize your efficiency by turning the AC on when going downhill and off when going up. I’m sure that will save gas but I’m not sure you will be able to measure the amount saved.

Yep, if the air conditioning is on, regardless of the temperature setting, it will be cycling on and off continuously. If you shut it off until the car warms up, the compressor won’t run at all. I do the same thing sometimes. Just kind of hate to run it and add heat to it for comfort. Today though at 101 degrees, it ran full time, and I used my remote starter to cool it down before I got in again. Never got too cold.

I have never seen an appreciable difference in fuel mileage with the AC off. Even fleet vehicles that have had AC failures and run for a week without it had no noticeable change in fuel consumption.

AC is for comfort. I’m with Rod Knox and really don’t care which is the most efficient. It’s a dehumidifier too so running it with mixed air is still a comfortable setting. I set it as close to the desired temp on the car we don’t have climate control on and don’t over think this thing.

Leave it running; just adjust the temperature dial to get the right level of cooling. Your driving style affects fuel consumption much more than the A/C setting.

I have a couple of acquiantances who operate both the heater and the A/C in this manner:

In the summer, they run the system at the highest fan speed, and they set the A/C temp for the coldest setting.
In the winter, they run the system at the highest fan speed, and they set the heater temp for the hottest possible setting.

In both cases, they keep the system running as above until the interior of the car is uncomfortably cold (summer) or uncomfortably hot (winter), and then shut the system down completely until the interior of the car gets uncomfortably hot (summer) or uncomfortably cold (winter).

The net effect is that you are only actually comfortable in their cars for a few minutes in the middle of these extremes, and most of the time you are extremely uncomfortable. With the presence of temperature controls, I have to ask…Why in the world would somebody do this?

At home, do you set the A/C to 65 degrees, wait until your wife screams “uncle”, and then shut the A/C down until the house reaches 85 degrees? At home, do you set the furnace for 85 degrees, wait until everyone is ready to pass out, and then shut the system down until the house is frigid? If you don’t operate the HVAC system at home in that manner, why would you operate the HVAC in your car that way?

With a little simple experimentation it should be possible to find a setting for the A/C that allows you to ride in cooled, dehumidified air that is consistently comfortable. This isn’t exactly rocket science!

set it and forget it


On most cars, with the fan on low and the A/C set to "normal" (outside air) it's pretty hard to get too cold...
...unless you're driving in the rain or at night, in which case you can get too cold on the lowest setting.


If you’re driving while it’s not raining with the fan on low and the A/C set to “normal” (outside air), and you get too cold, shut off the air for a while.

If you’re driving in the rain, keep the air on, with the fan on low and the A/C set to “normal” (outside air), and you’re too cold, give it a little heat do the conditioned air will keep the windows from fogging up.

What you never want to do is run the air on recirculate (the max. setting) with the air conditioning off. Humidity will build up, and I once read in an owner’s manual that it might damage the system. It might not be the case anymore, but there’s really no reason to chance it.

Lastly, on your next car, get something with climate control, where you set it on a certain temperature like a household thermostat. That way, you won’t have to worry about this particular problem.

We are having an awful summer here in Ireland. As in bad by Irish standards, And Petrol hit €1.68 a litre, so while it has not hot, it has been humid (misty windows) and saving money has been an issue.

I run the A/C on Eco mode and open the window if it is not raining and I’m going slow. I do not notice the instantaneous fuel consumption to dip when I flick the A/C on.

I turn it off fully when possible though…

I see a couple of you guys don’t live in the south east. Around here, when it rains, you turn on the defrost. If your defrost doesn’t automatically turn on the AC, then you push that button on as well, otherwise you will have trouble seeing where your are going. It would be nice for those “pop up showers” if there was a second bi-level position on the HVAC for dash vent and defrost.

Oops, make that a third bi-level position. There is already a low/dash and a low/high (windshield). We need a dash/high as well. Any of you car manufacturers out there listening?

I’m sorry, I just can’t help myself. Its not really the MPG. It just seems wasteful to run the AC and turn the heat up to make it comfortable. I tried this afternoon when it was 85 out but I ended up shutting it off for a while and then turning it on again. I walk around in the dark too when its only me around. I’ll waste plenty in other ways though so it must be something deep in my subconcious.

The way it works I would not be surprised if it did not show up at once when you turn on the AC. Overall it will.