Is it more efficient to leave your car idling?

Illuminating answers to environmental questions.
May 27 2008 4:25 PM

Is an Idle Car the Devil's Workshop?

Turning off your engine to save energy.

I always idle my engine while stuck in traffic or waiting at the drive-through. My wife insists that the greener move is to turn off the car every time we come to a stop, but I think she's nuts—doesn't restarting a vehicle waste a whole lot of energy? I remember learning that each restart burns the same amount of gas as idling your car for 30 minutes.

Illustration by Robert Neubecker. Click image to expand.

The Lantern assumes that you started driving way back in the heyday of the carburetor, when engines started up with a big gush of fuel. But unless you own an automotive dinosaur, your current engine is so efficient that idling would rarely, if ever, be an earth-friendly choice.


Today's cars use electronic fuel injectors, which rigorously control the amount of gas delivered to the engine when you hit the ignition. As a result, virtually no fuel is wasted during startup, and only a thimbleful is burned as the car roars to life. So forget about the 30-minute axiom you were raised on—the threshold at which it makes more sense to shut off rather than to idle should be expressed in seconds, not minutes.

How many seconds, exactly? A lot of environmental organizations advocate the 10-second rule: If you're going to be stopped for more than 10 seconds, it's best to shut off your engine. The one exception is when you're stopped in street traffic—it's illegal to kill the engine in many states, due to concerns that switched-off cars are more easily rear-ended (especially if an absent-minded driver forgets to restart once the gridlock abates).

At first, the Lantern figured the 10-second rule couldn't possibly be legit—surely it's an invention of auto-parts companies for whom worn engines are a boon. But he's slowly come around to buying it, in large part because of this field experiment by the Florida section of the American Society of Mechanical Engineers. The researchers concluded that restarting a six-cylinder engine—with the air conditioner switched on—uses as much gas as idling the same car for just six seconds.

Idling is similarly wasteful in frigid temperatures. Contrary to popular belief, cold-weather drivers needn't warm up their cars for longer than 30 seconds. The best way to raise an engine's temperature to optimal levels is to drive it almost immediately after startup; according to a study by the Ontario Ministry of Transportation, a car driven for 12 minutes in 14-degree-Fahrenheit weather will achieve the same temperature as one that idles for 30 minutes. (However, it's best to avoid rapid acceleration during that 12-minute warm-up drive.)

Frequent restarting does create some extra stress on your battery and ignition mechanisms, though probably not as much as you'd think. According to a study by Natural Resources Canada, obeying the 10-second rule will add roughly $10 to a driver's annual maintenance bill. How does that tab stack up against your fuel savings? Well, let's go with the conservative estimate of the Ohio Air Quality Development Authority, which states that the average idling car consumes about 0.156 gallons of gas per hour. (The Lantern has seen other much higher estimates but can't vouch for their veracity.) If you're able to cut out 10 minutes' worth of idling per day, and you need to restart your car an additional four times per day as a result, then you should save around 8.9 gallons of gas a year. At today's gas prices, that amounts to $33.74 annually, leaving you with well in excess of a double sawbuck.

OK, so cutting out idling, though certainly advisable, isn't going to pay for your kids' college tuitions. Nor, for that matter, will it make a huge dent in our national carbon footprint. Keeping in mind that burning a gallon of gas creates 19.564 pounds of carbon dioxide, if every one of the nation's 196 million licensed drivers reduced their idling by 10 minutes per day, we'd cut our annual CO2 output by 15.48 million metric tons. That would represent about 0.2 percent of the carbon dioxide that was emitted in the United States in 2006.

But if we were able to eliminate idling in stop-and-go traffic, the effect could be more dramatic. Right now, it is imprudent (and often illegal) to cut your engine while on public streets. There are automated systems, such as in the vaunted Toyota Prius, that can rapidly turn engines off and on when the car is, say, stopped at a red light or involuntarily "parked" on a bumper-to-bumper freeway; just apply some pressure to the accelerator, and the engine springs back to life. According to the learned folks at Car Talk, the widespread adoption of such technology could reduce our national fuel consumption by as much 10 percent.

Is there an environmental quandary that's been keeping you up at night? Send it to, and check this space every Tuesday.



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