Is gas mileage affected by your air conditioner?

A while back, I was involved in a conversation where a person was convinced that the air conditioner in their car affected their gas mileage.

They were so certain of this that they only run the AC when they absolutely have to. (Which for me is most of the time).

So, first a little info….  The AC compressor in most cars is powered by a drive belt (V-Belt, serpentine and sometimes, even a cogged style belt). The belt may power only the AC compressor, or it could be turning the power steering pump, alternator or other devices. Regardless of whether the AC is turned ON, the belt is still in motion. It runs most frequently from the main shaft of the car, and also may have an idler pulley that it contacts.

Let me say this again – Regardless of whether the AC is turned on, the belt is moving the AC compressor’s pulley. When you turn the AC on, a charge is sent to a clutch, the clutch engages, and that pulley that is already being turned transfers power to the AC compressor.

However, the contrary to what most people believe, the compress doesn’t only run when your air conditioner is on. The compressor is engaged when you turn on your defroster, and sometimes it is automatically turned on or cycled by the car’s computer, just to keep it lubricated. Other cars do not have an “AC button”, and instead have a temperature control that automatically cycles between the AC and the heater, to maintain a consistent temperature in the vehicle.

With all that out of the way, I set out to determine if there was indeed a difference in fuel economy when NOT using the AC.

I started with a baseline measurement….  Several months of closely watching my gas mileage, and then picked a month that I could bear to not run the AC to perform my test.

While I was at it, I figured I would give it every possible chance to save gas, by completely changing my driving habits.

The car is a 2004 Mazda 6 wagon, with a V6. It has slight modifications to it, including a high performance computer mod, which gives it a considerable amount of additional power on demand over stock.

Here’s the gas mileage from 2012-7-17 until 2012-10-18:

2012-07-27   20.1 MPG
2012-08-08   21.3 MPG
2012-08-22   22.0 MPG
2012-08-30   20.7 MPG
2012-09-14   20.4 MPG
2012-09-27  21.1 MPG
2012-10-08   21.0 MPG
2012-10-18   20.6 MPG

Pretty consistent actually…   My routes are similar each day, and the blends and octane of the fuel I use have remained pretty constant.

That’s an average of 20.9 MPG over those 8 tanks of fuel.

Now, my experiment…   Again, the goal was to maximize fuel economy.

The conditions:

• Do not use the AC, under any circumstances, not for defroster, not for cooling…  If it happened to come on automatically as a result of the car’s management system, fine, but don’t manually activate it
• Do not let the car idle – shut it down if sitting for more than a minute (because idling cars consume fuel, right?)
• Coast down hills, do not accelerate suddenly, etc (Hyper-milers do these things all the time)
• Wash the car, insure tires are at recommended pressure
• Drive UNDER THE SPEED LIMIT….    Yep, this was a difficult one. At no time did I exceed the speed limit. I paid careful attention to the posted limit, and drove under. Sometimes as much as 5 MPH

Okay, so this is a total change in my driving habits. I always keep it cool in the car, 68 degrees or colder. The AC is always running.  And I am not saying I have a lead foot, but, I don’t normally drive under the speed limit.

The result:

21.4 MPG, which is right in line with the average MPG I had been getting.

So next, I wanted to go back to my evil ways and run that air conditioner, not wash the car, drive it like I stole it (Okay, I drove it a little bit more aggressively than I NORMALLY drive), let it idle every chance I got…

What kind of gas mileage did I get when driving it that way?

20.0 MPG on the nose….

So in conclusion, I am convinced that at least in MY car, the mileage is pretty darn locked in, thanks partially to the poor aerodynamics of a wagon, and mostly to the fact that modern vehicles have very efficient air conditioner compressors, they consume very little horsepower/torque when they are engaged.

While there was a slight increase in fuel efficiency with the AC off, it is hardly enough to worry with.  A whole 6 or so miles extra per tank, but driving slow and being miserable doing it is just not worth it to me. Though, I could take the dollar and some changed saved and use that for better antiperspirant, because it is just too darn hot here in the South to go without AC!

The driving habits seem to have very limited affect too.  I even suspect my car gets BETTER gas mileage at 60mph average than it does at 55.

I will keep further track of this and might make a follow-up at some point.

What are your experiences?