found this on my home page today.
Save Gas

Be cool about keeping cool. If you really must park in the sun, don't jack up the air conditioner as soon as you get back in the car. Drive for a bit with the windows down to let out the excess heat first. Once you're at highway speeds -- 55 miles an hour or more -- driving with your windows down actually consumes more fuel than using the A/C because it creates so much aerodynamic drag.

MythBusters busted this one this season. At highway speeds, air conditioning always consumes more fuel. Granted, the number of data points they took was small... I'd like to see this experiment repeated with more scientific rigor.

Just had the A/C fixed in the Neon. I've been happy to have it back, even though my mpg just went down a few notches. It's in the high 90s here and I like having cool feet (and the rest of my body too).

QUOTE (zymurgist @ Aug 12 2005, 12:29 PM)

MythBusters busted this one this season. At highway speeds, air conditioning always consumes more fuel. Granted, the number of data points they took was small... I'd like to see this experiment repeated with more scientific rigor.

They also got conflicting results. They did it once at higher speed, using a computer with airflow meter. That one said AC and windows up was better.

The second time they lowered the speed (because the SUVs didn't like going that fast around the track) and switched to the less accururate (IMHO) method of emptying both tanks and pouring in equal amounts of gas, then running them dry.

Clearly the aeirodynamic effects will be more dramatic at higher speed. Also, the amount of fuel still in the tank when the engine stops running can vary between "identical" vehicles. I would like to see them do the test again with the airflow meters, and eliminate any driver/vehicle/whatever differerences by swicthing testing windows/AC between the vehicles. At various speeds, and with various types of vehicles too. Am I asking too much?

it isn't really that difficult...

1) use an OBD-II datalogger* to obtain microliters/injection (instantaneous fuel consumption).
2) find a nice section of straight, flat interstate.
3) set the cruise control.
4) open windows, close windows, AC on, AC off, whatever.

repeat as needed to obtain experimental error confidence bounds you like.

i happen to be partial to This One.

a few weeks ago while driving thru AZ @ 2am, I found that at 75mph our Hyundai Elantra got 40mpg with the A/C off....with the A/C on, it dropped to 37mpg....I didn't roll down the window since my wife was sleeping

bondo,

It would be nice to see the experiment repeated with the airflow meter, more data points, and another set of vehicles (say, a Camry or Accord or other popular sedan with better Cd than an SUV).

But I'm all about saving money on gas, really. Driving the Mercedes on my long commute was killing me. Even though the Neon is a shitbox by comparison, I'll take a 35 mpg shitbox with working A/C and CD changer (new Harry Potter book!) over my 18 mpg (on premium unleaded!) Teutonic luxo-barge, at least for a daily driver.

I liked the statistic, 1% fuel milage difference per 100 pounds of extra weight. If you're driving a 914, 100 pounds should make a lot more difference than if you had an extra 100 pounds in your Suburban...

Time is money and gas is cheap in this country...I drive pedal to the metal all the way!

Windows down, windows up aero is also going to change dramatically from car to car. An A/C compressor is certainly going to reduce economy, sure. On some cars, the additional drag of the windows down v. windows up isn't going to matter enough to even measure. One car, one test isn't going to bust any myths as far as I'm concerned.

Cars with a sunroof that can be tilted up in back, or rear windows that pop out, can offer excellent ventilation through the car's normal fresh air vents with a lot less drag, too.

Just put an egg between your right foot and the gas pedal.