Idle?

gasoline
fuel-economy

#1

Could anyone tell me the approximate time that a car needs to idle before it becomes cheaper to just shut it off? I’ve been scouring the Internet for a few days, and I’ve seen everything from every single stoplight to any stop more than 10 seconds… I’d appreciate any help or advice! Thanks!


#2

Probably the best source of information on fuel usage during idling would come from the generator manufacturers. Their figures for the fuel use of an unloaded generator motor, of the approximate size to an automobile engine, would be that type of data.

I saw a figure, somewhere, that an automobile fuel usage, at idle, would be 1/4 of the average mileage use. In other words, if the car used 2 gallons of fuel in an hour, which it would at 30 mpg, at 60 mph, then, at idle, that would be 1/4 of 2 gallons = 1/2 gallon per hour. So, 1/2 gallon divided by 60 minuets = 2 ounces per minuet.


#3

I have no good answer, but I will mention that I sometimes do shut my car off in certain situations - e.g. at a train crossing (while waiting for the train, not in front of it); dead stop traffic jams; waiting for the wife to come out the grocery store - stuff like that.

But I always balance the decision to shut down against the belief (knowledge?) that a large amount of engine wear occurs at start up. The little bit of fuel usage may be worth saving the wear and tear - I personally want a stop much longer than 10 seconds before I shut down.


#4

You should also consider that the various components of the starter circuit have a definite life span. Seems like I read somewhere that a typical starter has around 5K starts in it. Not to mention the ignition switch, which can and does wear out. It would be interesting to know average starter life in cabs, UPS trucks and the like.


#5

That’s a good idea! I know it’s different for each type of vehicle/engine/etc.

I wonder approximately how much fuel it takes to start the engine…


#6

I have thought about that, but the more expensive gas is, the more sense it makes to shut off the engine quickly. The fuel costs are going to top just about everything else in terms of importance pretty soon!

As for the starters in those vehicles, don’t they usually idle for a good bit of time during the day? I know that whenever I see a UPS/FedEx truck, they’re either driving or idling. I don’t know that I’ve ever seen one shut off.


#7

“I know that whenever I see a UPS/FedEx truck, they’re either driving or idling. I don’t know that I’ve ever seen one shut off.”

This may vary, depending on state regulations. In Central NJ, I constantly observe UPS trucks that are shut down for each stop, and are then restarted, only to drive to the next house, where they are again shut down. I have also observed that the local delivery trucks of the US Postal Service are shut down when the carrier goes up to the door of a home in order to deliver a package.

I haven’t observed what FedEx does because most people in this area have learned not to use FedEx after many bad experiences with them. If there is a choice, I always opt for UPS. (UPS trucks are corporate owned, whereas FedEx drivers have to purchase their own trucks and pay for their own fuel, and in effect, are contractors. Perhaps our local FedEx drivers provide worse service than those in other areas, but they are really bad in my area.)


#8

The most useful information to this familiar question comes from the first post, offered by hellokit. First I have to offer a correction. There are 128 ounces to the US gallon (let’s hear it for the metric system!) so 1/2 gallon per hour comes to almost exactly 1 ounce per minute.

When gasoline sold for $1.28/gal, a single ounce cost you 1 cent. Triple the price to $3.84/gal (we’ll be there next month) and a minute’s of idling time costs you $0.03 in fuel. That amount is invisible to anyone’s budget.

Now I wish I hadn’t done the calculations! I’ve always shut off my engine for those brief stops, and now I truly see how futile it really is in terms of penny-pinching. This is particular so if you try to factor in the mechanical costs of restarting each time.

So it pays to shut down the engine if you wait for a passenger to hop out and run an errand or when traffic comes to a standstill for a long delay. But for stops under two minutes or so, shutting down the engine merely provides emotional satisfaction rather than measurable savings.