Gas Mileage

[verbinfusion]

http://www.straightdope.com/classics/a2_393.html

I actually saw this exact same question answered on the Discovery Channel show, Myth Busters. They did two tests, the first determining that AC and windows up was more efficient, but the second test concluded that windows down was better. I tend to believe that windows down is better, but maybe that’s just wishful thinking.

[The_New_Guy]

They got pretty heavily criticized, though, on their own message board. For safety reasons, they had to limit the speed at which they conducted the test, IIRC to 45 mph. Many felt that the effects of drag would be much more pronounced at higher speeds.

[yerba_buena]

I’d guess that the amount of drag that open windows have varies wildly between different shapes of windows, how far they’re open, and the shape of the car around the window. I’m also guessing that air conditioning units aren’t standardized, either, in terms of power drain.

[SCSimmons]

Here’s another group that tested this theory, along with some other gas-saving theories. Their net conclusion: any savings from using electric A/C instead of 4-60 cooling (that’s four windows down & go like 60, for those unfamiliar with standard automotive abbreviations) are negligible compared to other mileage-improvement strategies they tested. If you want to save gas, minimize your acceleration & deceleration rates (by using cruise control when appropriate, and looking ahead to anticipate stops & starts when not), and keep your tires properly inflated. Do that, and you won’t care whether your A/C should be running or not.

[flodnak]

@The New Guy

They got pretty heavily criticized, though, on their own message board. For safety reasons, they had to limit the speed at which they conducted the test, IIRC to 45 mph. Many felt that the effects of drag would be much more pronounced at higher speeds.

If I’m remembering the episode correctly, there was another major flaw, too. They kept the air conditioning on full blast for the entire run. No big surprise that burns a lot of gas - fortunately it’s rarely, if ever, necessary. Running it at the level that people use on an ordinary summer day (a few minutes on the highest setting to get the car cool, then down to a lower setting to maintain the temperature) would have given a much more accurate test.

[Foaming_Cleanser]

@flodnak

They kept the air conditioning on full blast for the entire run. No big surprise that burns a lot of gas - fortunately it’s rarely, if ever, necessary. Running it at the level that people use on an ordinary summer day (a few minutes on the highest setting to get the car cool, then down to a lower setting to maintain the temperature) would have given a much more accurate test.

As I understand it, a car air conditioner is fully on or fully off because its compressor spins or it doesn’t spin. There is no in between. By turning the air conditioner down, you are mixing the cold blast with warm air through the heater. Turning the temperature down does not save energy unless the fan is switched off, and then only minutely.

Since the test was to discover whether less fuel was burned with the air conditioner turned on, or turned off with the windows down, turning it off intermittently could prove nothing.

[The_New_Guy]

Yeah, I agree. I’m pretty sure the a/c dashboard control only regulates how hard the fan is blowing the air out of the vents, and like you said, if you change the temperature control, it simply adds heat from the heater core in addition to the a/c. Neither has anything to do with whether the compressor is engaged. The compressor clutch cycles on and off depending on the temperature of the coolant in the lines, doesn’t it?

[Foaming_Cleanser]

This auto repair site discusses it, beginning near the bottom under Pressure Regulating Devices.

A friend of mine had a '41 Cadillac that looked like this. His wasn’t air conditioned, but he said the condenser on those that were filled the space between the headliner and the roof, and a huge compressor was in the trunk.

Page 2 of the Cadillac site says:

The '41 model year also saw the introduction of air conditioning as a Cadillac option. Truly, this was a big step from the classic era into the modem age – even though only 300 cars were equipped with the bulky apparatus that occupied a considerable amount of space throughout the car. In fact, the unit had no automatic clutch and could be disengaged only by removing the belt in the engine compartment. By comparison, Packard had advertised its “Weather-Conditioner” as early as February 1940, and it was every bit as bulky and primitive as the Cadillac arrangement.