Don't Sweat the Gasoline
Shutting Off Your Car’s Air Conditioner Saves Gasoline
For years our elders have counseled that if we really want to save gasoline, we should drive with our car’s air conditioner off. Roll down your windows instead goes the popular wisdom. You may eat a faceful of truck exhaust, but you won’t have to gas up at every exit.
It’s an issue worth thinking about because by some estimates around 5.5 percent of the gasoline burned annually by America's cars and light trucks, or 7 billion gallons, is used to run air-conditioners. That's equivalent to the total oil consumption of Indonesia. Just four states—California, Arizona, Texas and Florida—account for 35 percent of that extra fuel consumption.
MPG Stands For Most Perplexing Guesswork
Here’s what others have to say about the matter:
- The U.S. Department of Energy says that “operating the air conditioner on ‘Max’ can reduce MPG by roughly 5-25% compared to not using it.” But that’s a pretty big spread, and it makes the statement somewhat less than definitive.
- A 2004 study for the US Society of Automobile Engineers showed that at speeds above 56 km/h winding down the windows is preferable to switching on the air conditioning.
- Consumer Reports, on the other hand, wrote that their tests on a Toyota Camry show that A/C use had a negligible impact on mileage. “Using air conditioning while driving at 65 mph reduced the Camry's gas mileage by about 1 mpg,” the magazine said. “The effect of opening the windows at 65 mph was not even measurable."
- An investigation by the Florida Solar Energy Center reached a different conclusion with a Volkswagen GTI, this test found that at 108 km/h the increase in consumption was 3 per cent with the windows down, whereas with the air conditioning on it was 12 per cent.
- The Straight Dope newspaper column tested the theory by driving a Pontiac 6000 500 kilometers at an average speed of 60 mph. That car got 12.4 kilometers per liter (35 mpg) with the A/C off and the windows up, 12.2 km/l with the windows up and the A/C on, and 12.0 km/l with four windows down and the A/C off. Not a big difference!
Cooler Heads Prevail
It’s true that using your car’s air conditioner requires your engine to produce more power and consume more fuel. But air conditioners today are super-efficient. They create a lot of cooling using a lot less energy than they once did. By most accounts, running the air conditioner in a late model car reduces gas mileage by just a mile per gallon or so. (Some tests have found the impact to be as high as four mpg.) At the same time, today’s cars are streamlined aerodynamic wonders that have been computer-designed to slip through onrushing air with as little resistance as possible.
And that’s the crux of the matter because turning off our car’s air conditioner means opening its windows, which greatly reduces its aerodynamic profile and dramatically increases the drag created by air resistance. When this happens, your engine also works harder.
Older cars have inefficient outdated air conditioners that require more engine power than newer systems. These cars are also not as aerodynamically shaped as today’s digitally-designed autos. So if you roll down the window, you won’t have as great an effect on drag since your drag is probably already pretty sizable to start with. Thus using the air conditioner is probably going to create a more noticeable impact in an older vehicle.
In new cars, the air conditioner is super efficient and will cool without needing much extra energy from the engine. At the same time, the efficiencies attained by today’s hyper-streamlined body designs will be noticeably compromised by open windows. In these vehicles, it’s probably a tie. Air conditioning use may even yield the higher mileage.
The faster you’re driving, the truer this statement becomes. At highway speeds, the general consensus is that it’s better to operate your A/C than to roll down the windows. At lower speeds, like those found in urban settings and stop-and-go traffic, you’ll save more gasoline by turning the A/C off and opening the windows instead because the issue of air resistance is negligible. Some smaller cars may save gas by shutting off the A/C at any point because a greater percentage of the power produced by their smaller engines is devoted to the air conditioner when it’s on. For their part, tests show that hybrid vehicles get 15-27% lower fuel economy with the air conditioner running. There an open window may always be best.
A Conclusion That May Drive You Crazy
As you know, we decided to apply the powers of science and look into the issue ourselves. The result was our experiment in Joshua Tree in Joshua Tree National Park, which seemed to suggest that there is some truth to the idea that A/C use uses more gasoline.
Though Sara and Dave did save an average of 4% of the gas their cars were using by rolling their windows down, the fact is that there are too many variables involved to call our experiment conclusive. Your own experience may vary considerably depending on what you drive and where and how you drive it.
The truth is that it’s very hard to generalize about whether or not people should roll down their windows or use the A/C. Some should do the former and some should do the latter. Broadly speaking, older cars will probably use more gas running their older A/Cs and suffer less from whatever drag is created by open windows. Newer cars with more efficient A/C/s and computer-designed aerodynamics should probably roll up their windows as they ramp up their speed.
And that’s our simple rule of thumb: when your speed goes up your windows should, too!
