Gas usage question


#1

My daughter tells me that it takes more gas to turn the engine off (for a few minutes at the “fast food” drive thru) and then on again, rather than letting the engine sit and idle for the same few minutes. This does not make sense to me.


#2

Your daughter is wrong; at least, if you are referring to modern fuel-injected cars.


#3

I would have to do the math regarding how much extra load the engine pulls as it replaces the battery charge that it took to crank the car, relative to idling with a fully charged battery, but I am pretty confident that your daughter is wrong.


#4

On a modern fuel injected car, it doesn’t take any extra fuel to start an already warm engine, so your daughter is technically wrong. But the amount of fuel you save is miniscule and you’re putting a lot of extra wear on the starter and battery, so in the long run it might well cost you more in more frequent repairs. Also, in the drive-thru it is polite to keep the engine off so you’re not exposing the workers to any more exhaust fumes than necessary.


#5

I think this old urban myth came about because of the true fact that a cold engine uses more fuel than a warmed up engine does. Then someone assumed that your engine instantly becomes ice cold the second the engine quits running and so restarting it is making it warm up all over again.

No, it’s not true for modern fuel injected cars, nor was it true for cars with carburetors.


#6

I would have to do the math regarding how much extra load the engine pulls as it replaces the battery charge that it took to crank the car, relative to idling with a fully charged battery, but I am pretty confident that your daughter is wrong.

I seriously doubt that you’d ever have enough information to do that kind of calculation. Especially since it’ll be different for every single car out there…not to mention different for the same car but months apart…plus you have to take into account the altitude…and outside temps at the time. There are far too many variables.

Even still…the battery is NOT going to loose that much power even when it’s -20 to cause you to burn more then just letting it idle for a few minutes.

On the other hand…you are putting more wear and tear on the the starter and electrical system.


#7

Your daughter is talking out of… she is wrong.


#8

What if your number eight in line at the drive thru?are you proposing eight on/off cycles? If your the car at the window maybe,but not at the ordering speaker and not at the payment window maybe at the food pick-up window if your order is taking a long time. Now a diesel truck thats a different story I have had them ask me to turn my truck off at the ordering speaker because they cant hear over the engine noise


#9

You are correct and she is wrong. This is on the radio quite a few times.


#10

Everybody has these old daughter’s tales. A warm engine uses almost no extra fuel to start. Those cold engines use more until they warm up a little.


#11

Rather than going through the drive-thru, why not park and go inside and eat?


#12

For what it’s worth, my motorcycle, a 2001 Kawasaki ZRX1200 motorcycle with a 1.2 liter four cylinder engine usually gets about 44 to 46 mpg while commuting to work. On my last tank, I got 54.96 mpg just by eliminating all the unnecessary idling at red lights and while coasting up to them. I also went down a lot of the longer down grades engine off and clutch pulled in.
The amount of fuel an idling engine burns is significant, in fact I think that a large part of a hybrid’s higher fuel efficiency comes from shutting down the ICE whenever it is making zero horsepower.


#13

Why not save all the fuel and some money and stay home to eat?


#14

You can do that, how?