Q. Does letting a car idle waste more gas than shutting off and restarting the engine? If so, at what point, time-wise?

A. Just 10 seconds of idling uses as much gas as restarting your car, according to the California Energy Commission Consumer Energy Center, which recommends you shut off your car when you’re parked (at a drive-through business, for instance) for more than 10 seconds. Allow a car to idle for two minutes, and you’ll use as much fuel as you would driving one mile. That’s noteworthy, considering the average driver lets the car idle for five to 10 minutes per day. And besides wasting gas, excessive idling can damage cylinders, spark plugs and the exhaust system.

Several years ago, Edmunds.com did a comparison of various gas-saving measures. To test the cost of idling, testers drove two cars for 10 miles—they stopped 10 times and idled for two minutes each time. Then testers drove the same route but turned off the engine and restarted at each of the 10 stops. They estimated nearly 20 percent fuel savings on the restarted cars.

Sid Kirchheimer is the author of “Scam-Proof Your Life” (AARP Books/Sterling).