Auto A/C Question

Factual Questions
1

My first car, a 1970 Chevy Vega, had factory air conditioning installed and I remember using it during the hot summer months and it significantly reduced my gas mileage, but in those days gas was cheap so I didn’t worry about it much. I assumed the large gas mileage hit was due to the fact that it had a relatively small engine, a 2.3 L straight 4 cyl. aluminum small block if I recall correctly.

Flash forward to today. I have a Toyota Tundra TRD full size truck with a 5.7 L large block V-8. Even without the A/C on it gets poor gas mileage, 16-18 miles per gallon if I am lucky, and gas is no longer cheap. Based on my experience with smaller vehicles I only use the A/C when I really need it to keep my gas mileage up, but I’m wondering if because it has such a large engine the drag on the engine when the A/C is on would be negligible compared to what it was on my 1970 Vega.

So the question is, all things being equal, should there be a noticeable drop in gas mileage on my Tundra if I have the A/C on compared to not having it on, or should it be imperceptible? My intuition tells me that the difference shouldn’t really be noticeable, and I can run the A/C without seeing much of a gas mileage drop, but I haven’t been able to test my theory by doing an actual comparison.

2

Until someone comes along with a better answer, there’s a very old and inconclusive Straight Dope column What gets the better gas mileage: windows up, AC on, or windows down, AC off? (as well as the slightly more recent Does using your car radio reduce your gas mileage?)

3

I recall Car & Driver doing some testing, as well as another source I don’t recall. The bottom line, for modern vehicles, is that at any significant speed the air drag resulting from having windows open outweighs the air conditioner load. So, if you are driving at any speed over, say 30 MPH, you might as well roll the windows up and use the AC. If you are putzing around, you might save a little by rolling down the windows, if you don’t mind sweltering.

4

So the big engines don’t sweat having the A/C on’ doesn’t hold water. I should have figured as much.

5

The Mythbusters did this test and found that a car with its windows open ran longer than a car with the A/C on doing a run to empty situation. Which is opposite conventional wisdom!

6

I’d go with your intuition. Shouldn’t be a huge drop in mileage. Not enough to worry about at least.

7

I drive 100 miles each way to work and back and never notice any appreciable difference with the air and without the air: windows rolled up.

So, 200 miles a day in a 2l Hyundai Elantra and no perceivable difference either way.

I do notice a drop in mileage in the winter when the weather gets down below, say -20 C.

8

I’ve noticed this too in cold weather. I always figured it was due to the snow tires on put on, but it may be because the engine isn’t able to get up to normal operating temperatures.

9

It’s not the tires. I’m thinking oil viscosity, bearing grease viscosity, but there’s got to be more to it; perhaps incomplete combustion of fuel at low temperatures?

I’m not sure, but it is noticeable in the 10%age range. In city driving it could mean up to 25%, at least on some of my older vehicles.

10

Every year I take a 700 mile trip to Wendover Nevada from Los Angeles. If I drive alone I use no a/c and windows cracked or down, if the girlfirend goes it is a/c windows up all the way. I get the same mileage.

11

I think the much worse fuel mileage in the winter has as much to do with “winter formula” gasoline as it does with colder weather n though I wouldn’t deny that cold temps mean longer warm up for your engine.

12

Could it be because colder air is much more dense? More dense air will require more fuel injected to maintain the correct fuel/air ratio.

13

Which would mean your throttle should be less open to keep a constant speed and power output, so no.