Fact or fiction: Turning on the AC while idling will save gas.

Dear Car Talk

Dear Car Talk | Dec 01, 1992

Dear Tom and Ray:

I have been told by an automobile manufacturers certified mechanic how to conserve on gasoline consumption. He has practiced the proceedure for years and I am now following it. Please confirm that this is valid. When I am held up in traffic at long lights or stalled traffic and the car is idling, I immediately turn on the air conditioner. On my car this reduces the RPM with no change in throttle plate setting or manifold pressure. In other words, the added load of the AC reduces the RPM with no additional gasoline being used. Of course, some modern cars may change throttle plate setting to maintain idle RPM when the air conditioner is turned on, but some older cars like mine do not.

TOM: Gee, John, this is great. By why stop there? Why not maximize your gas savings by installing five or six air conditioning compressors? You'd have to modify the hood to fit them in. But you could probably bring your rpm all the way down to 150. Think of all the gas you'll be saving then. And at traffic lights, you could run pipes into the cars next to you and air condition them!

RAY: We're teasing, John. Like many promising theories, this one sounds good....BUT, it's actually wacko.

TOM: I hate to break it to you, but you're not saving any gas. You say in your letter that turning on the air conditioner doesn't change the throttle setting. That means you're not using any MORE gas. It also means you're not using any LESS.

RAY: When you turn on the air conditioner in your car, the same amount of gas is consumed by the engine. But some of that engine power is now being used to run the air conditioner, and because of that, there's less left over to turn the engine. That's why the engine runs a little slower.

TOM: It runs slower because the gas is being used elsewhere, not because less gas is being used.

RAY: And by the way, modern cars actually use MORE gas when the air conditioner is on. Modern cars have sophisticated engine management systems which compensate for the power drained by the AC. To make up for this loss in engine power, they automatically kick up the idle, which, of course, uses more gas. So on modern cars, your technique for "gas saving" is completely counter productive.

TOM: But don't feel bad, John. I've bought into my share of wacko theories over the years, too. For instance, for a couple of years, I believed that my brother knew what he was talking about.

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