Scenario: Manual transmission on steep downgrade.
Should I put the gear shift in neutral or keep it in gear with my foot off the gas?
I have heard that with the car in gear and foot off the gas that fuel is cut off to the engine. However, if I keep the car in gear the rpms are anywhere from 2000-3000 (with my foot off the gas)
If I put the manual transmission in neutral my rpms are just under 1000.
To me it seems the lower the rpms then the less firing off the cylinders and less gas used. But I remember hearing that keeping it in gear while going downhill shuts off fuel to the cylinders even though the rpms are 2000-3000.
Anybody know?
This forum really really reeeeeally needs a sticky for this because it gets asked so often.
Here’s the way it goes down: just because your engine is spinning at a particular RPM, that doesn’t mean that you are using the same amount of fuel. For instance:
Scenario 1: you are rolling down the road at 60 mph with your foot at 30% throttle and turning 2500 RPM. You are hardly using any fuel.
Scenario 2: you are moving up hill at 60 mph with your foot at 60% throttle and turning 2500 RPM. You are using much more fuel.
Scenario 3: you are moving down hill at 60 mph with your foot at 5% throttle and turning 2500 RPM. You are hardly using any fuel, probably a negligible amount.
Generally, the less that you’re pushing the throttle, the less fuel the engine is using. Leave the car in gear because if you have to slow down a great deal or stop while traveling down hill, your brakes will be cooked quickly if it’s in neutral.
It depends on the car. Some cars will totally shut off fuel when the engine is turning over faster than idle speed. Mine does that. Most do not, so they are using the same amount of fuel in gear as not. The difference is very small.
In either case you want to think of safety first. If you are using brakes more because of this, then you are also wearing brakes and that also cost money. If the hills are long and steep, then you are also risking overheating the brakes and that can kill you.
As an added note, the safety experts believe that it is enough of a risk that some states have laws against putting your car in neutral in situations like this.
I understand that it also can damage some automatic transmissions.
Depending on the car there is an ideal (from fuel usage standpoint) time to have your car in neutral and when to have it in gear if it is one of the cars that shut off the fuel supply when in gear. Frankly it is not worth worrying about for real world driving. I have a car that does shut off the fuel, so that works out best for me under almost every situation.
Have you done any math on this? Do you imagine the better method, whichever that may be, is actually worthwhile in terms of savings? Those folks who feel comfortable crunching numbers estimate you may save up to three cents per month. (This was before the sudden rise in gasoline prices. Now you may be saving six cents per month.) Yet the folks who take pride in such economy methods continue to overspeed, burning away their miserly monthly advantage in a few minutes.
Conclusion: Do whichever method gives you the most satisfaction. But understand it really has nothing to do with true fuel economy.
Having the car in gear is more efficient and it is safer. Even though the RPMs are high, the car’s momentum is turning the air conditioner compressor, the alternator, the water pump, the power steering pump, and any other accessories that are belt driven. This means that your momentum is also powering your electrical accessories. If any fuel is being pumped into the pistons, it will be the same or less than when the engine idles while coasting. So coasting won’t save you any gas. The biggest danger of coasting down a long steep grade is that the brakes will overheat and you will loose control. Taking that chance isn’t worth it.
Thanks- - -good points.
I was wondering though how an engine can continue to run if fuel is totally shut off to the engine?
As responsible advisors on this forum we have to tell you that this driving style (coasting in neutral) is illegal in many states and is dangerous, while saving virtually no gas.