Debate continues on whether running the air conditioning or driving with the windows down uses more gas. The general consensus is that open windows decrease fuel economy more than a/c does, but only at highway speeds.
Why Air Conditioning Lowers Fuel Economy
Air conditioning uses engine power, which is why it results in the engine using more gas. And the hotter the exterior temperature, the harder the air conditioner works to keep the interior of the vehicle cool. The engine then must work harder too.
Why Open Windows Lower Fuel Economy
Driving with the windows down reduces the aerodynamic quality of vehicles and creates air resistance, or drag, from the wind that enters through the windows. That requires more engine power to propel the vehicle forward.
Which Is Better?
Experts generally say that on the highway, driving with the windows open uses more gas than having the a/c running.
However, Discovery Channel’s MythBusters team conducted an experiment that didn’t support this. With two identical sport utility vehicles on Altamont Raceway in California moving at the same speed (45 miles per hour), they found that the model with the a/c running ran out of gas before the one with the windows rolled down did. In fact, the SUV with the open windows ran a full 15 miles further than the other one did.
The study’s results are intriguing, but they also are lacking. The results could be different for:
- other vehicle models
- higher or lower speeds
- the size of the open window space
- the air conditioner setting
- the exterior temperature
Speeding along a highway at 70 miles per hour with the windows down might use more fuel than running a/c would.
In fact, the Society of Automotive Engineers did its own study and found that the effect of open windows on SUV gas mileage was much less dramatic than with full-size sedans. That’s because SUVs aren’t aerodynamic in shape.
Both vehicle models in the study had V-8 engines. It’s still unclear exactly how these issues affect smaller, sleeker cars without that level of horsepower.
In general, you’ll save more money on gas with the windows up and the air conditioning on at highway speeds. Turn off the a/c and open the windows when you’re driving at slower speeds if it’s not too uncomfortable. That will save money and delay any needs for air conditioning service due to wear and tear over time.