Which wastes more gas? Running your car's air conditioner or driving with your windows down and fighting the wind?

There are variables, say the experts and the seat-of-the-pants advisers, the greatest being driving speed and the aerodynamics of your vehicle.

The higher the speed, the more wind resistance affects mileage. So, you may be saving gas by driving with the windows open and the AC off at 25 mph. But at 55 mph and faster your vehicle's aerodynamics are increasingly degraded by open windows.

At highway speeds you may get better mileage with the windows up, but it may still not be better than it would be with the AC off.

"Poking around city traffic, aerodynamics doesn't come into the picture very much," said auto air-conditioning guru Ward Atkinson. The auto engineering consultant worked on some of General Motors' first auto AC systems in the mid-1950s. He's a regular speaker on climate control at Society of Automotive Engineers confabs.

While wind resistance from running at highway speeds with your windows down may eat up more gas than running windows-up with the AC on, there is no escaping the fact that "it takes energy to cool a car," Atkinson said.

You're going to pay to stay cool. How much is the question.

An SAE study conducted at the GM Proving Grounds and in a GM wind tunnel on a full-size car and a large SUV showed that increased wind resistance from running with windows down at more than 55 mph cuts gas mileage by 20 percent, while running the air conditioning cuts gas mileage by 10 percent.

But at urban speeds, rolling down the windows is cheaper than AC.

The crossover point is about 40 mph.

What Atkinson recommends is making your AC use as effective as possible.

Here's how:

1 Use the "recirculate" or "inside" air settings so you're cooling air that's already been cooled, rather than struggling to cool the furnace blast coming in when you set it to "fresh" or "outside."

2 Turn down the fan speed once you reach a comfortable temperature.

Other widely accepted practices include opening a rear window or a front and a rear window when you first start up a car that has been cooking in the heat, to give the AC a boost. Roll the windows up as soon as the temperature inside is cooler than that outside, and also select the "recirculate" setting at that point.

Check your car's cabin air filter. A clogged cabin air filter (most new cars have them, and they're relatively cheap and easy to replace) can dramatically weaken your AC system's ability to cool.

Bob Belman said he doesn't hear a lot of debate about windows down versus AC, even though he makes a living out of servicing auto AC in southern Arizona.

Belman said people come to him because they want to chill, now. They've made up their minds.

There's no question that when you roll down your windows at highway speeds "there's going to be some drag. But I don't have any data," he said.

What he does know is that auto AC units are generally more efficient and reliable than 20 or 30 years ago.

Some efficiency improvement is due to variable-displacement compressors, which have become fairly common on modern vehicle AC systems, Atkinson said. They change the size of the compression area depending on demand, so it's not just a matter of the compressor running full blast or not at all, as was the case with older systems.

Atkinson has a practical approach to the question: It feels good and it doesn't cost that much. But if that doesn't do it for you, he said, "It's worth a couple miles per gallon" to avoid being distracted, uncomfortable and annoyed.