Low pressure tires

Mythbusters used just about the worst possible circumstances to test that particular hypothesis. They used Full-size SUVs with big V8 engines, and they did the test at IIRC 45mph because of the sharp corners on the test track they used. Silly really. The idea was to see if driving with the windows open used more gas than running the A/C. On a big SUV at 45 mph, it was a wash. On a properly aerodynamic car at a realistic highway speed, it would not be. Modern A/C compressors are extremely efficient, while driving with the windows open at 75 mph royally screws up the aerodynamics. And of course, as speed increases, the relative percentage of the engines power needed to run the A/C compressor decreases. Takes the same amount of power to run the compressor at idle as it does at highways speed.

Kevin Rhodes Westbrook, Maine

My 2003 Jetta diesel gets about 3-5 miles per gallon better mileage in the spring and fall then it does in the summer. I have windows open but not wide open in the spring and fall. AC in the heat of summer. No myth busting there. I don't worry as much about crummy mileage in the summer but if it is crummy in the fall and spring I am lookng for reasons. My 84 GTI used to lose about 10% when running the AC. About the same as the mileage loss on the Jetta as it could dip down to 45 mpg.

Could seasonal fuel variations have anything to do with it?

-- Mike Smith

No. Drive a week with the AC on and mileage goes down about 10%. Next tank it will go back up with no AC. I could flip back and forth as the weather does. Fuel mix does not flip but the temps do. We had a few days of frost already and then 70+ days a few days later.

I do not know for sure how much the summer/winter diesel mix affects mileage. I do know straight B100 chicken fat base causes about a 5% or more drop in mileage but better smelling exhaust. I have not run soy bio in a long time. I do not know how it affects mileage.

Oh, THAT myth! Okay. There's no doubt in my mind that running the AC uses much more fuel than any losses due to aerodynamics with the windows open. No way that windows could possibly cause that much drag. That is the conclusion that Road & Track reached in a recent test, too. As someone else mentioned here in this thread, there's a clear-cut correlation between using AC and loss of fuel mileage, but no correlation between having the windows open and losing mileage. Does anyone feel the car slow down or hear the engine start to pull a little harder when they lower the windows? Of course not. But you do feel that momentary bump when the AC compressor kicks in.

That one really is a myth. AC costs fuel, pure and simple.

Actuarially it appears to depend on the car, including the engine and transmission used as well as the speed.

To tell you the truth, I don't worry about it. I keep the windows closed (almost all the time) and use heat or air as needed. I like being comfortable and the worse mileage I have ever had with my TDI is mid 40's so I guess I can afford it.