If you’re stuck in traffic or waiting in a drive-through line, is it better for the environment to idle your car engine or turn off the gas? According to Slate, unless you’re driving something out of the dark ages, it’s rarely a good idea to leave the car running if you’re going to be stopped. Today’s cars use electronic fuel injectors, which rigorously control the amount of gas burned when you turn on the ignition. The outdated axiom that says turning on your car wastes the same amount of fuel as leaving the it idling for 30 minutes went out with the carburetor.

Check out Slate’s take on the 10 Second Rule here, which says that if you’re going to be stopped for more than 10 secs, kill the engine. Find out why it’s not always legal in some cases, and how the rule stacks up against fuel savings in this age of astronomical gas prices. You can also takepart and learn more about hypermiling and increasing your fuel efficiency here.

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