A couple weeks or so ago, our kids had a friend over to play. When his mom came to pick him up, she left her car running in our driveway. After we stood at the door chatting for 10 minutes or so, my wife said something like, "you don't have a diesel, you shouldn't leave your car running." The mom then said that she'd heard that it was better for the car/cheaper to leave it running than to turn it off and then back on after such a short period of time.
I actually thought it was the opposite -- that you could even shut your car off at stoplights and it would save you money -- but what do I know about cars? (I also thought it didn't matter if it was a diesel, that it should still be shut off, but again, what do I know?)
So since I was unsure, I thought I'd ask all of you out there. Anyone have any insight into this issue?
The answer one would think is common sense is in fact common sense. The amount of gas used as a "burst" to start your car is negligible and only equals to a few seconds worth of idling.
Google "car idle 10 seconds" and you will find many sources.
I used to hear people say it's cheaper to leave lights on when you leave a room if you are coming back soon because turning them on and off takes more energy. This always seemed rediculous to me.
The rule of thumb is actually quite simple:
Using energy uses more energy than not using energy.
Posted by: Apex | June 05, 2009 at 10:31 AM
http://green.yahoo.com/blog/climate411/71/when-to-turn-off-your-engine.html
Also according to this "GREEN" source, no warm up period is needed for your car in the cold, idling for extended periods can do more damage to your engine than starting and stoping, and idling a diesel can use up to 4 times more fuel than when you are driving a diesel (that one seems counter-intuitive but who knows).
Posted by: Apex | June 05, 2009 at 10:37 AM
Yeah, back before there was precisely controller EFI (electronic fuel injection), it really WOULD take a lot of gas to start a car -- but not any more. As another poster said, the amount of gas used for starting a modern car is roughly equivalent to only a few seconds of idling. Strictly speaking, it would be more efficient to shut your car off at stoplights! Although not very practical of course. That's why BMW (and others) are getting ready to introduce an automatic cutoff system that shuts the engine down when you're stopped with your foot on the brake and then starts it up again when you start to ease off -- kind of like the auto on/off of hybrids like the Prius, except the new cutoff systems will be on non-hybrids too. This will put a greater load on the electrical system to run all your accessories while you're stopped, of course, but apparently the engineers are convinced it can handle it.
Posted by: Matthew Rogers | June 05, 2009 at 10:47 AM
These people are probably correct (shut off the car to save gas), but I would like to see Mythbusters test this.
Posted by: Paul | June 05, 2009 at 10:57 AM
In 1995 the CarTalk guys discussed it, and thought it was a choice between wear and tear and gas.
http://www.cartalk.com/content/columns/Archive/1995/November/12.html
I would bet the answer depends on the model, age and state of repair of the car.
ASME Florida Section attempted to quaa/ntify the issue, with a complication of A/C here:
http://www.sections.asme.org/florida/ASME%20Fla%20Section%20Virtual%20Mythbusters.html
Posted by: Dru | June 05, 2009 at 11:00 AM
I think this was a myth buster thing.
Basicalyl if you aren't going to be starting it right back up, turn it off.
Posted by: ken | June 05, 2009 at 12:08 PM
the time length varies by car but i've always heard that anything under 30 seconds is better to idle. anything over 30 seconds turn the car off. i think the 30 second number accounts for possible additional wear and tear not just the gas. and this 30 seconds was before modern fuel injection systems. that 10 seconds someone else posted might be the right length of time now. either way over 1 minute is way to long to idle.
Posted by: John @ Hard Work Blogging | June 05, 2009 at 12:18 PM
10 minutes! What a waste. Think about it this way, idling makes your car go 0 mpg.
I think that goes to the other end of wear and tear too - it is not good for the engine to just sit and idle for extended periods. Engines are designed to work best under load, so having it just sit there can cause incomplete combustion of the fuel leading to buildup, especially if your car isn't brand new.
I can't find data on how much fuel is used starting, but on a fuel injected car it isn't much more than normal, more like stepping on the accelerator a bit hard for a second. The consensus seems to be 10 seconds of idle = 1 start. Starting works the starter, and the battery, so if you do it too much those would wear out faster than they should. That is why the start-stop systems mentioned above have more powerful starters and better batteries. But you would have to start and stop an awful lot to worry about it!
Posted by: Chris | June 05, 2009 at 12:25 PM
Every minute you idle you're wasting around 1-2cents of gasoline.
Posted by: Jim | June 05, 2009 at 12:30 PM
I've seen people do that at convenience stores, too, if they just need to buy something quick. It's a good way to come out to an empty parking spot.
Posted by: Aron | June 05, 2009 at 12:32 PM
The "let it run" concept was used when cars used carburetors. That is just about the only time it is worthwhile.
Posted by: Michael | June 05, 2009 at 12:52 PM
10 minutes is definitely long enough to turn it off. the thresholds i've heard for when to turn it off have all been in the under 1 minute range
Posted by: Carrie | June 05, 2009 at 01:33 PM
If you look at the new cars coming out soon, they have a new technology that shuts the engine off when it the vehicle is stopped, i.e. at a stoplight. This would make it seem that it is better and more efficient to turn your car off instead of idling.
Posted by: Flinkster | June 05, 2009 at 01:38 PM
Has anyone considered that every time you start your car it puts additional stress on the starter and the fuel pump? It may save gas but wear out other parts prematurely/ And is it legal to turn your car off at a stop light??
Posted by: zOrv | June 05, 2009 at 01:52 PM
In response to Apex, the theory behind "should I leave the lights on or turn them off" or "should I leave the engine running" obviously has its basis in physics. As with all electrical and mechanical devices the power required to start a device is greater than the power required to keep it going. Determining how long you should leave it on depends upon the efficiency of the device in a running state and comparing the values over that period of time. Generally, we're talking a few seconds, maybe a couple minutes as with electric lights, so what Ken said is correct. If you aren't going to start it back up right away, turn it off.
Posted by: Raleigh | June 05, 2009 at 01:59 PM
I'm pretty sure the wear and tear on your starter from restarting your car once is negligible compared to the gas you waste by idling for 10 minutes.
Posted by: S | June 05, 2009 at 02:27 PM
Funny how most of the posters above only think of the financial aspect of idling a car. First and foremost should be the environmental aspect: Polluting the environment and wasting a non-renewable resource.
Shame on her! If that was my driveway she would be told to turn that off immediately.
Posted by: Matt | June 05, 2009 at 04:59 PM
Wear and tear on the car starter is a realistic concern.
My last car's starter died after about 10 years. So figuring if I drove the car daily for 10 years thats about 7000 starts. Replacing the starter cost about $250. So thats a replacement cost of around 3.5 cents per start. Thats around 2-3 minutes worth of gas. I don't know if that math would hold up on average or for most cars but its probably in the right ballpark.
Posted by: Jim | June 05, 2009 at 07:02 PM
@Matt
This is a finance site. You might want to adjust your expectations to your audience.
Posted by: Apex | June 05, 2009 at 10:30 PM
Paul: "I would like to see Mythbusters test this."
I thought they did. If I recall, they made the same mistake as with their "turn off a lightbulb" episode: they calculated the cost of energy on startup (negligible) and concluded it was cheaper to shut it off and turn it on a few seconds later. But they forgot to account for the wear and tear on the starter/engine or the lightbulb itself. Turns out those concerns are far greater than the energy usage concerns, at least financially speaking.
If a starter is expected to last, say, 10,000 starts and replacement cost is $250-500 depending on the model, then it's anywhere from 2-4 cents per start, or about 1/100 of a gallon of gas. I would believe Jim's 2-3 minutes worth of idling on that.
Posted by: LotharBot | June 06, 2009 at 01:15 AM
When idling, the engine is not running at a high level of efficiency. It is best to shut off the engine if not using it for about 30 seconds, though that depends
You'll want to make sure that you are using top tier gasoline in addition to being smart about when you decide to shut off the engine.
Oh, and by the way, skip the drive through & go inside to pick up the food - it can be faster, and it will save you gas.
Posted by: Wise Finish | June 06, 2009 at 02:51 PM
Environmentalists used to argue back in the 1980s that you shouldn't turn off your car because starting it took several minutes of gasoline at once. Of course, this was back in ye olden dayes with cars less efficient than what we have today.
At any rate, I leave the car to idle in the winter up here in Canada...
Posted by: El Cid | June 06, 2009 at 04:22 PM
It just became illegal in NYC - you will be fined something like 1000$ if you idle for more than a few seconds!!!
Posted by: katie | June 06, 2009 at 09:07 PM
The 30 seconds rule is also what I have read about (on a different personal finance site). Although, I don't know where they got that info from, nor whether wear & tear are included in the tradeoff.
Posted by: F | June 07, 2009 at 02:28 AM
My '92 Blazer stalls if I let it idle for more than 30 seconds, so this isn't a question for me. If I'm stopped at a busy intersection or waiting for someone in a parking lot, off it goes. I wait with my hand on the key and haven't held up traffic yet.
Posted by: FekketCantenel | June 09, 2009 at 02:55 PM
well first of all you are all freaks and idling makes the air we breathe a hazardous pollution which could easily harm your health....
Posted by: Anynonmous | November 10, 2009 at 06:57 PM