General Question

timothykinney's avatar

Amount of time to leave the car running?

Asked by timothykinney (2730 points ) January 15th, 2009

I heard somewhere that it was cheaper in gas (not emissions) to leave the car running for 15 minutes rather than turning it off and turning it back on (to go into the post office, for example). Does anyone know where to find the actual number of minutes for which this is true? I’m sure it depends on the car, but I’m looking for a ballpark estimate.

If you know the amount of emissions generated by starting a car versus leaving it running, that would be helpful also.

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14 Answers

DrBill's avatar

The number is 1–3 minutes for car with Carberators it is near zero if your car has Fuel injection .

scamp's avatar

I think I would save a lot of money in gas if I left my car running while I went inside a store or the post office around here, because it would be gone when I come back out!!

tennesseejac's avatar

The gas difference is so insignificant that I wouldn’t worry about it. Today it is 8 degrees in Nashville so Im certainly going to leave it running.

timothykinney's avatar

@DrBill, do you have a reference for your estimates?

Knotmyday's avatar

A guy I worked with tried that tactic; he ran into a liquor store to buy a lotto ticket, and when he came out…he was suddenly a cyclist. With a new nickname: “Dumbass.”
Plus, the lotto ticket was a loser- double dumbass.

timothykinney's avatar

If you happen to drive an old enough car with an odd enough personality, you might find that you can remove the key from the ignition while the car is still running. This allows you to lock the doors and and lower the probability of the car being stolen.

I certainly wouldn’t leave a Ferrari running with the doors unlocked in Houston.

Snoopy's avatar

“Idling is the worst action a hypermiler can do. If the vehicle is running for more than 10 seconds, it makes more sense to shut the engine off and re-start it. It takes less gas and has very little effect on the performance of the car.”

From this link

EDIT agree this if for fuel injection vehicles only.

Also….consider wear and tear on the starter if you do this to excess…..

timothykinney's avatar

@Snoopy, Thanks for the link, but I was looking for something that maybe offered a calculation, rather than anecdotal evidence.

@Knotmyday, at least you still have your car. My brother had his Dodge Neon stolen twice in Dallas. Both times they entered the locked vehicle and drove it away without breaking a window. Security is an illusion.

timothykinney's avatar

Okay, I think I found some good evidence published by the American Society Of Mechanical Engineers.

It says that for a V6 engine, starting the engine accounts for 6 seconds worth of gasoline at idle. This would imply that turning off your engine is best for short stops. I am assuming this study was done with fuel-injection.

So there is still the question of a carbeurated engine.

Here’s another link with general information about energy myths I found interesting.

Cardinal's avatar

It’s better to turn the car off in Washington state. If the Gestapo finds a car running (for any flipping reason even to warm it up in your garage with the door open) they will ticket you up to $400.00 and can and may and have towed the car right out from under your nose! Asshats!!

timothykinney's avatar

I still can’t find any data for carbeurated cars…

DrBill's avatar

@timothykinney
My reference is myself. In college I helped run benchmark test on cars when fuel injection was the new thing.

Yes, I’m really that old, thanks for asking

Jack79's avatar

I have also heard this from professional drivers (bus drivers, truckers, taxi drivers). Apparently (especially for old-tech petrol cars) it is cheaper even for as long as 10 mins.

Even if this is true though, I could never get myself to do it, unless of course there were practical reasons (such as turning on the engine for 3–5 mins on a frosty morning to warm it up). I never keep it on for more than a minute if I’m stopped (even if someone has just popped out to buy cigarettes or something).

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