Saving Gas at Stop Lights

OK - a nerdy question. You come to a red light and you know it is a long light - say 90 seconds. Will you save gas by turning off the engine and restarting it just before the light turns green? Does the act of starting the engine use a large amount of gas? If so, how long must you be stopped where it pays to turn off the engine?

Part 2 - if you are saving gas and money, will it be offset by the starter engine failing prematurely?

I told you this was a nerdy question.

This has actually been talked about at length and even studied. You won’t save gas, but you just may create a safety hazard. You’ll probably get rearended.

Hybrids shut the engine off when not needed, but they have a different setup than you. They shut the engine off whenever not needed, not just at lights.

Part 2: yes.

Do you have any links to the studies? We have a bit of a bet here. The basic question is: how much gas is used to start an engine versus the rate of usage at idle.

We know its not safe etc… this is just theoretical.

Normally I agree with Mountainbike, but not this time. An engine that is shut off is NOT burning any gas, and you WILL save gas, and money, by shutting down the engine whenever possible. Hybrid vehicles automatically shut down the gasoline engine when the car is at rest to save gas. People in Europe, who pay WAY more than we do for fuel, do this regularly.

If you know it’s a long light, shut it down. Will the starter wear equal or exceed the fuel savings? Good question.

Also, why is it not safe? The engine will restart in seconds, and it does NOT take more fuel to restart a modern engine than it does to keep it running.

I don’t know of any official studies on this subject but any gas savings due to shutting the engine off would be less than miniscule.

However, even that tiny bit of fuel saving is going to be more than offset by the energy required to manufacture, ship, distribute, and pay for repeated trips to the parts store and tow truck fuel use due to the huge number of worn out ignition switches, neutral safety switches, starters, etc, etc. that are going to be worn out by the repeated cycling of the starter.

I should have added that an average starter will be good for X number of cycles. If you hit that starter 25 times a day instead of 2 or 3 what is going to happen? Obviously the starter is going to go south a lot quicker.

I recently read a discussion about this that surprised me. A knowledgeable poster indicates that park or neutral will use more fuel:

You get distracted like that and you end up hurting. It may be better to adjust your route. Ninety seconds is way too long to be stuck waiting. Most lights don’t keep you that long. I’d rather but a MIRT (mert?)at Radio Shack and make the light change. Don’t get caught. You want to get there safely. When off-duty policemen / detectives shoot you in a fit of road rage (provoked, and after a night out, the story said) you might not want to inspire Joe Ordinary. You will get mad at yourself for trying to move with the engine off.

Does anyone have any data about this? Forget about whether this is dangerous, save a little, save a lot. We are just trying to find out - what is the rate of consumption of gas at idle versus the amount of gas that is used to start the car?


GM actually built a car (prototype) that did just that. This was 10 years ago…I think if it saved gas there would be one out there already.

I think this is a no-brainer. Yes, shutting it off saves gas. Before fuel injection it was true that start-up used a lot of fuel, but with a modern fuel injected engine, it just isn’t true anymore.

You can easily get your own data on this. Fill up your tank. Then idle your car for 90 seconds and fill it up again. This will show you how much fuel you used. Then start the car and shut it off immediately. Then start it again and shut it off immediately and fill the tank. your bet will be settled by how much fuel it takes to fill the tank. Make sure you use the same pump to dispense the fuel each time. Also, when you fill the tank, use the same setting on the nozzle to be sure that the fuel is being dispensed at the same speed each time, preferably at a slow speed so that the pump doesn’t get shut off from splashing gasoline.

Yes, this will wear out your starter sooner than if you didn’t do this.

My advice, however, is that unless your car does this automatically, don’t do it manually. There is a safety issue. There might be an extremely rare circumstance where driving away quickly might be in your best interest. Suppose you see in your rear view mirror that a large pick-up truck has its brakes locked and it is sliding toward you. If your car is started, you might have a chance of moving out of the way. If you have to start it manually, you are going to spend precious time starting the engine and putting it in gear. What if an ambulance is trying to get though?

Considering how much fuel you will save, it isn’t worth it.

Okay, I concede, in the average modern 4-banger you’ll probably save a gallon of gas every 20 years. I’m really reluctant to agree to call that “saving gas”.

In my humble opinion based on where I drive you’re just likely to get rear ended. When the light changes, everyone is primed to go. And half of them are only half paying attention. They’re operating on “automatic” mode. I still don’t think it’s a great idea.

Looking at this from an Economist’s standpoint…

You have to consider the opportunity cost of shutting down your engine, in other words - the implicit value of the next best alternative not just how much “extra” gas you burn.

  1. You risk being un-able to get out of the way quickly, should the need arise.
  2. What if your engine decides it doesn’t want to restart for whatever reason ? Now you’re stuck at an intersection and have to call the tow truck. Ouch.
  3. A new starter will cost you $400-$500 i’m sure, so the fuel “savings” will be trivial in the long run, if you ask me. It seems like starters are a common wear & tear item even under normal conditions. Why risk it ?
  4. On a hot day shutting down will mean no more A/C and if you forget to put your window’s down you have to turn on the car once again and then put them down and by the time you accomplish that you’re just sitting there stewing in summer heat and taking in fumes from people smoking cigarettes and car exhaust and even if you do shut the car down as soon as the engine stops the light will probably turn green anyway so why bother ?

I hope this makes sense.

In countries where gas is expensive and urban pollution is severe there are a number of cars availble that shut themselves off after a few seconds. I’m talking about Japan. It can be a long wait there to get through an intersection. I shut the engine off at railway crossings (we have long trains here), and in traffic jams. At long traffic lights I put the car in neutral (good for the transmission); it also puts less load on the engine and releases fewer greenhouse gasses. The extra wear and tear on the starter from constantly using it probably wipes out any gas savings.

Yeah I agree with turning off the car while waiting at a RR X-ing, unless it’s hot as hell and I need the engine to be running for the a/c.

I wouldn’t be surprised. Anything goes, car manufacturers used to do that. AFR 11.5:1 at idle is too far from its standard, though.

Hope nobody is doing that anylonger, otherwise they’ll have Greenpeace volunteers carrying a placard in no time.

less than miniscule.less than miniscule.less than miniscule. i agree. totally.

your question offers the answer. “Will you save gas by turning off the engine and restarting it just before the light turns green?”

you are planning on being psychic?

where is the efficiency? what is the cost or enviromental savings?

also: a hybrid has the ability to speed (relatively) out of harms way even if the gas engine is off. (the electric motor does the work)

a traditional auto, if shut off, has no ability to get up and go.

this seems unsafe to me.

An engine that is shut off is NOT burning any gas, and you WILL save gas

 Well not always.  In older cars that squirt of gas from the car will out weigh the savings.  In most newer cars it will reduce usage, but then there is the additional drag to place the electrical power used to restart the car.  

 In real life it is just too small to worry about and the balance may tilt either way depending on the car and the time. 

People in Europe, who pay WAY more than we do for fuel, do this regularly.

 Maybe, but I have not noticed it, nor does that mean it really works, nor does it mean it does not wpork.  

  I will agree that there is little or no safety issue, but there can be some inconvenience to other drivers when you add up all those few seconds delay.  

  Overall I suggest there just is no real issue related to gas, but I would suggest that the wear on the starter and the possible aggravation to other drives if you are a little slow off the line makes it a bad idea.

Bottom line: Nobody knows. This question could be answered with some apparatus with gasoline in a sight glass safely refillable and viewable by the driver and passenger. When Chevrolet’s Chevy II became available, they held a contest to see who could get the best gas mileage and used equipment as described.

If engine stopping at stoplights was an effective way to save gasoline, it would be a simple matter for a vehicle mfr to offer an engine shutoff option via pushbuton that would restart the engine on pressing the accelerator pedal using electronic rather than mechanical throttle control which is available now in some models. Starters can be upgraded to withstand the usage. As of now, nobody offers this option, likely because it is not realistic. There we go again with an opinion based on speculation!

The safety issue is a red herring, in my view. It is speculation and as was stated, does not address the OP’s question.

Green means “go”, not “start your engines”.
Envision driving through a city where everyone shuts their engines off at stoplights. Do you really think traffic would flow smoothly?

I still don’t think it’s a good idea.

1 Like