Driving with the windows up and the air-conditioning on gives a car better gas mileage than rolling the windows down -- or so we've been told for years. But despite this advice, car owners continued to report that their miles per gallon (MPG) actually dropped when driving with the AC turned on.
What's the best way to improve your car's MPG on hot summer days -- AC or windows down? Look at what the experts say and you'll understand how your car's gas mileage changes on warm-weather trips -- the answer may surprise you.
Bad Gas Mileage Is a Drag
Your car's drag is the amount of resistance it has to the air moving across it as you drive. Cars that have a shape that's long and lean (like a canoe) have less drag -- and usually get better gas mileage -- than a car or truck with a big, boxy shape.
Driving with the windows down does change the amount of drag on a vehicle -- but only a little bit. A lot depends on your car's size, shape and speed. If, for example, you drive a big SUV, rolling your windows down won't really alter your car's air drag much, since it already has so much drag. But if you drive a small, sleek roadster, rolling the windows down will increase your drag by a larger percentage, like around 20%.
The other important factor is your car's engine, and how much work it has to do to run the air-conditioning unit. Little 4-cylinder engines have to really crank when they're also running an AC unit.
A big 8-cylinder engine, which you'll usually find in larger cars, trucks and SUVs, can run an AC unit without breaking a sweat.
AC or Windows Down: What the Experts Found
Auto experts from Edmunds.com and the Society of Automotive Engineers (SAE) have run tests to see which strategy -- windows up, AC on or windows down, AC off -- yields better mileage.
In an oft-cited SAE study from 2004, engineers compared two large vehicles' fuel efficiency -- an SUV with an 8.1 liter, 8-cylinder engine, and a sedan with a 4.6 liter, 8-cylinder engine -- under each driving condition at low, medium and high speeds.
Both vehicles got the best gas mileage when their AC was off and the windows were rolled up (this is, of course, only an option on cooler days). When the windows were rolled down, fuel efficiency dropped, especially for the sedan with lower drag -- it didn't affect the SUV as much, because the SUV already had a lot of air drag. And when the air-conditioning was turned on and the windows rolled up, fuel efficiency was at its worst, despite the vehicles' big engines.
The folks at Edmunds.com were surprised when they found similar results with a pickup truck, which got almost 10% better gas mileage with the windows down and the AC off when driving at 65 mph. Edmunds noted, "the air-conditioner produced a measurable drain on the engine and a resulting drop in fuel economy ... [perhaps because] the aerodynamic qualities of trucks are more similar to a brick wall than a sedan."
Finally, a survey from Consumer Reports revealed that, overall, "Running an air conditioner at 65 mph can reduce your fuel mileage by about 1 mpg and possibly more if you drive faster."
It's unfortunate that the SAE test didn't include smaller cars with lighter 4-cylinder engines, that the Edmunds test didn't examine a range of vehicles under different driving conditions, and that Consumer Reports doesn't have more detailed info. Hopefully we'll see some better research in the near future, but in the meantime ...
Gas Mileage and AC: What You Can Do in Warm-Weather Driving
Simply put, running the air-conditioner always reduces gas mileage. Always -- at least according to the experts.
So, the most common-sense advice is to drive with the air-conditioning off and the windows open whenever possible, especially in stop-and-go city driving. Cracking a few windows and putting your car's fan on high -- without running the AC -- will usually help you stay comfortable by moving air through the vehicle on warmer days.
When you're going on a long, high-speed freeway trip, it might be quieter and more comfortable to roll up the windows and turn on the AC -- though that will inevitably bring down your MPG. In particular, cars with smaller 4-cylinder engines usually get much better mileage with the AC off, even at highway speeds with one or two windows open. And in any vehicle moving at highway speeds, cracking a window a few inches will usually provide plenty of airflow without increasing drag significantly.
Of course, it also helps to dress for the weather -- it's nearly impossible to keep your cool when you're wearing a wool business suit and a necktie during a heat wave.
The best way to figure out what works best for your car is to try each situation yourself, since vehicles are all different -- what saves gas in a Mini Cooper might not be the most fuel-efficient strategy for a Cadillac Escalade. Here are some other easy tips on getting better gas mileage.