Many tipsters eager to squeeze the maximum number of miles from each gallon of gas have weighed in on this question, and a few groups have even played out the different scenarios on test courses and under myriad driving conditions. Not surprisingly, the results vary. Here’s what we know for sure:

Gas mileage is best when the AC is off and the windows are up. Why?

First, using the air conditioner consumes energy, which means you’ll sacrifice a few miles per gallon by turning it on. Estimates of the decrease in gas mileage range from 1 to 4.5 mpg.

Second, driving with your windows down creates extra air resistance, which can reduce your fuel economy—especially at higher speeds, and especially for more aerodynamic cars.

The fuel efficiency of sedans, for example, is largely due to aerodynamics. Anything that increases wind drag for a sedan will likely decrease mileage. The impact on the aerodynamics of an SUV, however, is much smaller with respect to fuel economy because most SUVs are boxy and not so aerodynamic anyway. Indeed, driving with windows down in an SUV, even at higher speeds, is a slightly better option than using the AC.

So what should you do?

In warm temperatures and at low speeds (less than 40 mph), drive with the AC off and windows down. The effects of wind resistance on fuel economy are usually less than the effects of running the AC.

In warm temperatures and at higher speeds (greater than 40 mph), roll up the windows and turn on the AC. The effect of wind resistance will likely approach (or exceed) the loss in fuel economy due to the AC, especially for sedans.

Also remember: If the AC is on, it’s better to recirculate cabin air than to let outside air flow in. In addition, low fan speeds are better than high fan speeds.