Very debatable - this subject..
I'd say idle no more than 2 to 3 minutes.
The Diesel however, uses so little fuel, so this - 5 minutes..
The engine and ambivalent temperatures are a factor..

For 5 minutes, definitely shut it off. I think the break-even point is about one minute.

I have often wondered about this. Here is hopefully some ROM perspective.

If you were driving 60 mph at 20 mpg that would use 3 gal/hr or 3/60 = 0.05 gal/min. So 5 minutes would use 0.25 gal, ie a qt of gas for 5 minutes (at 60mph).

Idling 5 minutes would be a lot less than that. How much?

Well when I drove big rigs (diesel) they told us to shut down if we had to idle over 5 minutes. Those outfits used about 15 gal/hr at speed (3-4 mpg then). I recall being told they used 1 gal/hr at idle = 0.017 gal/min or 0.083 gal for 5 minutes.

Assuming the gas burners were similar, burning about 1/10 at idle as at speed, your car would then burn about 0.1 x 0.25 gallons = 0.025 gal/5 minutes at idle.

This says nothing about the possibility of extra gas being consumed by the starting (or shutting down) processes. I don't know if extry gas is consumed during start/shutdown, maybe somebody here does.

Certainly extended idling doesn't do the cylinder walls any good and the air polution may be greater.

I'm guessing that shutting down makes you feel better more than anything.

.025 gal x $3.00/gal = 7.5 cents

I read before that some certain car engine (don't remember which one) was said to use about 1/3 gallon of gas per hour when idling. Looking at Mxyplx's example of .025 gallons in 5 minutes, that is equal to 0.3 gallons in an hour, which is almost the same as 1/3 gallon an hour. So maybe that figure is at least somewhat close to correct, meaning that the 7.5 cents for 5 minutes of idling is also about right.

I notice that UPS drivers always shut their truck off when delivering a package, which only takes a minute or so in many cases, and sometimes even less. All of the UPS delivery trucks I've seen have gas engines, so this would make their idling fuel usage probably fairly similar to a car with a gas engine. UPS studies all aspects of its operations extensively, so I'd bet that they've tried it both ways and found that shutting the trucks off uses less gas.

I agree they are on top of things but thot shutting down was some sort of safety procedure. Maybe not. Next time you see a UPS driver with 0.025 seconds to spare ask em. Or just trot along with em :-)

When cars had carburetors, it took more gas to start the engine than to let it idle for 3 minutes. But not so with the fuel injected engines of today. So shut it down. Besides, look at the hybrids. When you stop at a red light, the engine shuts off. And when you pull away, the engine starts. So if it were more fuel efficient to leave them running, they would.

Thanks everyone!
Christopher, I hadn't thought about the carburators versus the fuel injection. That's a good point.
I also think UPS shuts their trucks off for safety reasons. They probably ran over someone while it was parked and running.
But the big rigs I see parked all night, always leave their's running. Does starting one of those really use that much gas, to warrant leaving it run all night??
Thanks again everyone. I'm going to start shutting it off.

"But the big rigs I see parked all night, always leave their's running. Does starting one of those really use that much gas, to warrant leaving it run all night??"

I'm not a trucker, but I suspect that the big rigs running all night at a large truck stop are powering the amenities in the sleeper while the driver rests (TV, lights, AC/heat, etc.) I haven't seen any running all night when parked in a hotel lot.

also the big rigs have turbos in thier rigs, they spin upwards of 150000Rpm, and they get HOT, so I have heard they run for several minutes to let the still spinning turbo's maintain oil circulation while they spin down and cool. but the all night runners are most likely just keeping the a/c and such on. not to mention the refrigeration for the trailers.
John

They shut them off because if they leave their truck
running, someone could jump in, steal the truck and
packages. Also if the thief was involved in an
accident,UPS would be liable. If a driver left the truck
running and didn't set the brake and it rolled into
something or someone, the driver would be fired
immediately.

Gosh, if the big rigs leave the engines running for AC, TV, etc., I wonder how much gas that would use??

There are numerous ways to secure a truck whether it is or is not running, and it could still roll if it were shut off and the brake wasn't set or it was left in neutral. I don't doubt that security may be an issue as well, but I am confident that the truck will use less fuel if shut off during deliveries than if left running.

Semi trucks do use a lot of fuel to idle all night, which is why an increasing number of truckstops provide units that the trucks connect to which heat and cool the truck as well as provide electric power for TVs, coffee pots, and other plug-in items in the truck. In the not-too-distant future we'll see an end to the practice of leaving semis running all night. Some areas have already outlawed it.

An idling engine burns up fuel to go nowhere. The less of it you can do the better.

In a fact sheet titled, "UPS Uses Technology and Operational Efficiencies to Reduce Fuel Consumption and Emissions," UPS writes:

"Sometimes the biggest impact comes from the simplest step. At UPS, the importance of fuel conservation is demonstrated throughout the ranks with drivers playing an important role.

Idling: UPS drivers are trained to always turn off their package cars when they stop for a delivery, never idling at the curb or in a driveway. Even if the driver is out of the truck for a few seconds, the vehicle is always turned off."

Here is a link that might be useful: UPS Fuel Conservation Fact Sheet

"Gosh, if the big rigs leave the engines running for AC, TV, etc., I wonder how much gas that would use??"

Zero. They burn diesel. :-)
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Considering the truckers it might be interesting to compare the fuel cost vs motel accomodations to say nothing of the inconvenience and impractibility of same. Or impossibility of even obtaining a motel room. (Bitter memories here).
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UPS drivers must be racked with indecision when they encounter a 2 minute red light.
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Can't help wondering about UPS's trade off between fuel savings vs starter and battery upkeep/replacement. It wasn't mentioned directly.

**Idling: UPS drivers are trained to always turn off their package cars when they stop for a delivery, never idling at the curb or in a driveway. Even if the driver is out of the truck for a few seconds, the vehicle is always turned off."**

I'll have to ask the ups guy that stops by my work why they do this for sure. I was under the impression that it was so they don't get ripped off. I think they lock the doors too. I applied for a job in ups's shop back in the 70's. The guy I spoke to at that time said they go through a bunch of starters and ring gears.

During the winter the truckers leave the engine running because they burn #2 heating oil. Yes, boys and girls, that's what diesel fuel is. And under 10 degrees it gells up. Then those things are a bear to start. Now I know that most truck stops put kerosene in the fuel to lower the gell point but some truckers don't buy their fuel at the brand name stops. So they don't know what they're burning. So they leave them running, especially in winter.

I did ask a UPS driver.
I posted what he said.
I haven't seen any of the fuel saving UPS vehicles
around here. Just gas and diesels.

I'm sure a company as big as UPS can figure out a way to
save on fuel consumption. Whether it is always carried out
however, who's to know.

When deciding how much money you're saving by shutting down your engine, you might want to factor in the cost of replacing your car's starter, and increased wear on the battery, starter switch, ring gear, etc. If you're stopping for 10 minutes or so, yeah, go ahead and shutdown. But I sure wouldn't turn off my engine to save a minute's worth of gas.

It's like deciding if it's more economical to save the brakes by downshifting when approaching a red light: I can change brake pads and rotors myself. Changing a clutch sends me shopping for a mechanic.

Which is why I said to idle no more than 2 to 3 minutes. I will shut down when I see an "endless train"....
The starter,gears, and battery must be considered.

Why United Parcel does not run Diesel is beyond me - I'm certain this will be changed in the near future.
Diesels use very little fuel when idling...

Sure, you have to look at the whole picture. Gas is only part of the cost of driving. But with today's fuel injected engines, if you're just looking at gas usage alone, it's not going to take a very long shutoff time before you're ahead of the game as compared to leaving it running. In Europe there are some cars models available that shut the engine off when you're stopped in traffic. Since they pay $5 a gallon or more for gas, I'm sure they wouldn't do that if it actually used more fuel.

In that article I linked above, UPS describes some fuel cell and electric vehicles they're using in some places. I have often wondered why they use all those gasoline-powered trucks, too. I imagine that security is part of the reason why they shut those trucks off, so if that's the case, it's easier to start a gas engine 200 or so times a day than a diesel.

Thanks for the info guys. Very much appreciated.

As for the UPS debate that's started up, I guess my two cents would be that UPS could have that policy for any number of seemingly logical reasons. (Less gas, safety, theft, or image which is the first thing I thought. I remember a news article not two long ago about one of the democratic party leaders letting the limos that picked them up idle outside of the parliament building and people were pissed off. It doesn't matter to your average joe whether it's actually saving gas to leave it running, all they see is an unattended vehicle stopped and running.) As I was saying: They could have the policy for any of these reasons, and the problem is that it can be impossible to find the REAL reasons from these huge corporations, even when they give an explanation you still have to question it...

It also seems like there's never just ONE reason for policies like this. So i'd be interested to hear more of what ya guys find out.

-=Andy Pacer

The reason people shut off their cars in Europe has as much to do with lowering the global warming impact as it does with fuel cost. I try to turn off my car if I will be stopped for more than 20 seconds, including stoplights. The saving in emissions is worth it, even if the money is small. BTW, UPS drivers have for years tried to maximize the number of right turns and minimize the number of left turns they make on their routes, to save gas. I read somewhere that they estimate their fleet saves as much as 3,000,000 gallons of gas a year using this strategy alone.

If anyone has had to crawl under their car and yank out a starter and replace it, you might think twice about stopping and starting every minute. Not to mention the wear and tear on the teeth of the flywheel. Or paying somebody to do the above. I have replaced enough starters or starter contacts to hesitate when I feel the urge to stop the car at that red lite from hell... Just sayin...

Well I am sure replacing starting parts in an engine can be unpleasant if you do not enjoy auto mechanics, however I think the consensus has been that stopping and starting as opposed to wasting a precious resource is the way to go.
If you are still not satisfied:

Component wear caused by restarting the engine is estimated to add $10 per year to the cost of driving, money that will likely be recovered several times over in fuel savings from reduced idling. The bottom line is that more than ten seconds of idling uses more fuel than restarting the engine.

Source:
http://www.consumerenergycenter.org/myths/idling.html

Here is a link that might be useful: Source