Is it a myth that idling your car takes less fuel that shutting it off and restarting?

It's a myth because the entire time your car engine is idling it's using fuel; in contrast a modern fuel-injected car uses no fuel when the engine is stopped, so your fuel savings after a few seconds of not idling will add-up to something on most trips. The myth came about because old cars with carburetors could be hard to restart, this wasting fuel if the engine was not stopped for a minute or longer.

For electronically fuel injected cars that don't have an automatic start-stop function, in general if your engine idles for more than 60 seconds, you'd have used less fuel by shutting the engine off and restarting it again when you need it - that's how little fuel it takes to re-start a modern electronically fuel injected engine. If the engine has an auto start-stop feature then just let the system decide when it's appropriate to turn off the engine for fuel savings.

Keep in mind that the cost savings from shutting down and restarting the engine doesn't take into account additional wear-and-tear to the starter motor, so if you decide you want to be "that guy" who turns the engine off at every single traffic light that lasts 60 seconds or longer every time you drive be prepared for the starter to possibly wear out sooner than it otherwise would and cost you a little money eventually that way, but in general you'd use less fuel and spend less money (even with more frequent starter replacement) if you shut the engine off any time you know it will be idling for more than a minute. It's not especially practical to frequently and manually shut the engine down in the real world but it's true you'd save fuel on a modern car if you did it.

That idea came from the days of carburetors, when it took relatively a lot of fuel to start a car. With computer-controlled fuel injection it's a lot less. The time it took to burn more fuel than it took to start was at one time 20 minutes, then it was 10 minutes, now it's around 10 seconds!

But many states have laws against turning your engine off at a light or stop sign.

Here's an article about it from Slate

http://www.slate.com/articles/health_and_science/t...

Better to shut it off. Many new cars have a stop start feature that shuts off the engine at traffic lights - with modern fuel injection it doesn't a lot more on startup than it does at idle.

Depends on how long your idling. If you're idling for like 20 minutes then yeah shut the car off. But running into the gas station real quick, it is better to just leave it running. Not only for gas but stopping and starting your car frequently within short periods of time isn't good for it either.

It would depend on the time factor as well.

It depends on how long you idle.