Dear Dollar Stretcher,
In "Recession Proof Your Family Finances" by Diane Schmidt,she mentions never using the a/c in the car because it uses so much gas; there's a lot of confusion around this and I swear that I've read Click and Clack say that this isn't true; it's the wind drag caused by rolling down the windows that uses more gas; the a/c runs off the battery.  could you please clarify?   Thanks.

Debra 

Debra,
I'm a car nut, but not an engineer so this won't be too techinical, but I'm confident of it since I've read the same answer over and over. Here's what I've read over the years.

There's no one set answer for all situations. You're balancing two different effects.

First, the ac does NOT run off the battery. It runs off of the engine. And it does take some extra gasoline to use it. How much depends on the particular car.

Second, opening your car windows does create some extra drag. More drag requires more gas to overcome that extra resistance to pushing the car through the air. How much drag depends on the shape of the car and how fast you're going. NO drag when you're at a stop light. Lots of drag on the interstate.

So you're dealing with trying to compare two very hard to quantify numbers. What most experts say is that there's not a big savings to turning off the ac. So if it's hot enough that you need it, don't feel bad about using it. If you're super conscious about saving gas you might want to turn it off and open the windows if you're driving in town. You probably won't increase your gas mileage by much, but for some folks every drop counts.

Keep on Stretching those Dollars!

Gary