According to road tests, the fuel consumed by starting up a car engine is roughly equal to the amount consumed while idling for 10 seconds. In general, larger engines consume more fuel per second of idling than do smaller engines, but the 10-second break-even rule applies pretty much across the board. Also be aware that idling with your AC on wastes 50 percent more gas than idling with it off. So now you have a new five-second rule on hot days.

And the impact of idling goes beyond gas consumption. Excessive idling can harm your engine as well. An idling engine doesn’t operate at its optimal efficiency or temperature. The result is incomplete combustion of fuel and the deposit of corrosive residues on engine parts. Over time, these residues can reduce your engine’s ability to run smoothly, its fuel efficiency, and its longevity.

Don’t be worried that repeatedly restarting your engine will wear out the ignition or run down the battery. This idea became outdated once electronic ignitions became prevalent.

Of course, there are two types of idling: unavoidable (creeping rush-hour traffic on your main commuter thoroughfare) and avoidable (waiting for your total BFF to flounce down to the car so the two of you can go shopping).

Idling in heavy traffic, and at stoplights, is unavoidable—unless you drive a hybrid. Hybrid engines save gas by turning off automatically when the vehicle is not moving. But some experts suggest that you can save gas and reduce engine strain by shifting automatic transmissions into neutral while waiting at long traffic lights.

In many instances, idling is avoidable and unnecessary. Americans waste an estimated 3.8 million gallons of gasoline per day absentmindedly idling their engines.

Here are a few ways you can help cut down on idle fuel waste:

Don’t get warmed up. Break the habit of "warming-up" the engine before you drive by revving the engine. This practice should be relegated to drag-racing-movie scenes in which the competing heartthrobs give each other the evil eye from the starting line, or left to jerks who want to show off how many noise ordinances they can break with their muffler-free hot rods. Most modern cars can be driven safely after only a few seconds of idling time.

Avoid the drive-thru. Whether we're picking up fast food or coffee, depositing money in the bank, filling prescriptions, or getting married in Vegas, we just love to get things done while sitting in our cars. Idling in line at the drive-thru is harmful to your engine, your wallet, and the environment. Park your car and walk in, if possible. Otherwise, you'll miss hearing the Elvis impersonator sing Amazing Grace.

Turn off the engine when you’re waiting. Anytime you find yourself sitting for minutes with the engine running, just turn off the car. That includes times when you’re at a railroad crossing, in the parking lot leaving a concert or sporting event, at a valet stand, or waiting to give your aforementioned BFF a ride to or from home, work, jail, rehab, etc.

Remember the 10-second rule. It may seem trivial, but the cumulative effect can really add up. If you reduce your total idling time by only five minutes per day, you could save up to 20 gallons of gasoline and more than $60 per year.