When driving on the highway, most of a vehicle’s power is used simply to overcome aerodynamic drag. On the other hand, air conditioning also uses a lot of power.
Drivers are often told that keeping the windows rolled up, which significantly reduces drag, and using air conditioning actually results in better fuel economy than driving at high speeds with the windows open. In fact, CNN/Money has offered the same advice in a story we’ve run on our site.
In Edmunds.com’s test, conducted at a steady 65 miles per hour, “windows down” or “A/C on” made virtually no difference in mileage.
The Mustang got 4.1 percent better mileage – 30.7 mpg vs. 29.5 mpg – with the windows down. The Land Rover got 1.6 percent better mileage with the windows down, a difference of just 0.3 miles per gallon.
Basically, the extra fuel used by the air conditioner is made up for in improved aerodynamics at high speeds. Your not really saving any fuel but, at least on the highway, the A/C isn’t costing you appreciably either.
I don’t think it would do any major harm or burn up much more gasoline to run the AC with the top open. Cold air tends to stay low in the cabin anyway, so you really aren’t losing much cold air out the top opening in any event. You’re not cooling the whole outside, as the cold air will mostly stay inside the cabin.
Opening the windows would be worse. You’d lose more cool air that way. Opening the top is the preferred method if you want something open.
BTW, most AC’s have two settings, one which recirculates 100%, and the other which allows some new fresh air in. So if you use the fresh air setting, you are basically doing the same thing as keeping the top open.
If a car air conditioner works the same with the windows open or closed, why is it that your electric bill goes up if you leave the house windows open with the A/C on?
If you leave the house windows open with the air conditioner running, the air conditioner will run more because of the lack of containment of the cold air. The unit may work marginally harder because of higher refrigerant temperatures, but the big difference is that it will run more of the time constantly cooling fresh outside air.
House air conditioners turn on and off. Some car air conditioners turn on and off, too. But even if it doesn’t run constantly, the air conditioner in a car will run more, because the car is surrounded by glass, which lets in sunlight. It’s much more difficult to cool a car than it is to cool a well-insulated house with the windows covered. This is why there’s a smaller difference in efficiency with a car.
"If a car air conditioner works the same with the windows open or closed, why is it that your electric bill goes up if you leave the house windows open with the A/C on? "
The answer to your question is this: When a home A/C reaches the set temp on the thermostat, the A/C shuts down. On an automobile (most anyway) the A/C just keeps on going, and going, and going, even if it is too cold in the cabin. The way to control the temp is to turn up a little heat. Yes, heat from the heater. The compressor dosen’t give a flip if the windows are open, or the top down, or the doors off your Jeep.
Tardigrade was correct in his response on August 22.
patrick24 August 25 Report “Some cars have temperature settings; Presumably the compressor turns off or works less when the desired temp. is reached, much like a home air conditioner. For cars without temperature settings, the driver may turn down the fan or adjust the cooling setting once a comfortable temperature is reached, instead running it non-stop at the cold setting.” If a driver turns down the fan or adjusts the cooling setting, the compressor just keeps on chugging along. Adjusting the setting just mixes some heat with the cold air. (on most cars especially older ones)
On my '09 Caravan we can adjust either side, right or left but we are just letting in a little heat. It dosen’t save any fuel.
Missileman, on a hot day’s drive I personally enjoy opening my sunroof wide, cracking my windows and inch, and turning the AC on full. I like the cool air drifting over my body while the sun tans my face and the wind flows through my gray hair. Perhaps that makes me “a few bricks shy of a load”, but I enjoy fresh air and sunshine far more than I enjoy maximizing my mileage. Physics be damned, I would respectfully submit that those who compromise their enjoyment to maximize their milage are the ones who have made the compromise.
Missileman, it dosen’t cost more to run with the windows open than closed unless you consider a small wind drag. The compressor runs the same, either off or on.
Tardigrade August 22 Report In response to the caller who wondered if it was bad for her car’s AC to run it with the windows open. It seems to me that when a car AC is on, the compressor runs continuously and the cabin temperature is controlled by adding heat to the air. If this is the case, it wouldn’t make any difference if the windows are open or not, since the compressor runs continuously anyway Correct.
There may be some systems that shut the compressor off and on but most of them don’t. And I wouldn’t want it to, it would wear out the clutch in one summer.
And what school should I go to? I would like to see it in print.
You might find a good vocational school in your area that teaches automotive technology. I agree that the AC compressor will come on briefly when the defroster mode is selected. The AC is just acting as a dehumidifier in those cases to help clear your windshield. It only cycles on for a little while.
One suggestion for running a/c with windows open: With car parked on a sunny day, interior will get extremely hot – perhaps above 130-deg F, even if ambient air temperature is much less, say < 90. (That’s how babies and dogs die if left in a parked car.) So when first starting out, it’s not a bad idea to drive, briefly, with the windows open to clear out all of that hot, hot air quickly. Once done, close the windows to permit the a/c to work properly.
Joedcorn, what you just said is true, in my opinion, but it dosen’t address the argument whether the compressor works harder with windows open or not, I contend that it dosen’t. In all the vehicles I have owned since '66 the compressor ran all the time the A/C was on. Except for a little wind drag there is NO difference. Turning up the temp setting merely mixes a little heat with the cool air, it dosen’t shut down the compressor.
“Urban puzzle”: it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car’s drag).
Computer-based mpg measurements: * 11.7/11.8 with A/C on and windows up * 11.7/11.8 with A/C off and windows up * 11.3 with A/C off and windows down
So, according to the computer, it’s better to use A/C with windows up.
This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you’re-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).
Jamie’s A/C car ran out of gas first – Adam’s windows down SUV ran for another 30 laps – completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.
Mythbusted
Now if they would test for windows down and ac on low, med, high we could answer the mpg/cost question. My blazer gets 23 mpg highway, front windows up, front windows down 1/3, ac on or ac off, cruise control on or off. I like a little ventilation and a little cool,and have found no difference in mpg, even vs windows closed, no ac or defrost.