when going downhill with a standard transmission and the rpm’s increase, is more gas pulled into the engine if the throttle is not depressed? In case it does, I’ve been coasting with the clutch in to get lo rpm’s and using the brake if I have to slow down. Reasonable or flawed logic?
Most modern cars are programmed to cut the fuel in those cases. I also have a standard, and I leave the clutch engaged all the time. My 1988 Supra will cut fuel flow through the injectors when the throttle is closed, and I’m coasting downhill. Then, I have the added benefit of free engine braking to keep control, and not have to ride the brakes, wearing them out quicker.
There are two parts to the question.
Fuel:
Most modern cars cut off fuel when the engine is turning over at a higher RPM than idle. There for for that time you are not using any fuel. But you are using momentum, so you will be going slower at the bottom of the hill and may use more fuel going up the next hill. Which will save you the most overall? Likely going down hill in gear, but it is not certain.
Older cars don’t cut off the fuel so there is always a minimum amount of fuel flowing so in that case you would save the most fuel by disengaging the engine.
Safety:
Generally it is safer to stay in gear. Going down long steep grades and relying on brakes to slow you down can be both illegal and unsafe. Brakes can overheat and fail.
So all things in moderation is best for both safety and mileage.
The amount of fuel you can save is not all that big, although it is measurable.
It depends on the steepness of the hill. If it is so steep that you have to ride your brakes to keep a safe or legal speed, then it is best to leave it in gear and take advantage of the fact that most modern cars shut off all fuel during engine overrun.
If the hill is only steep enough to maintain the cruising speed that you would drive anyway, leaving the engine in gear means that you have to open the throttle enough to match the engine’s no load rpm to the rpm of the transmission’s input shaft in order not to slow the car with engine braking. Let’s say that speed has your engine running at 4X the idle rpm. If your engine’s idle fuel consumption is .3 gallons/hour, then a 4X idle rpm fast idle will burn about 4X .3 or 1.2 gal/hour. It takes a lot less fuel to make zero horsepower at 650 rpm than it does to make zero horsepower at 2600 rpm.
Is it illegal? maybe, let’s try an experiment. Let’s find a cop sitting on the side of the road and I’ll glide past him in neutral and you cross the double yellow to pass me while I’m doing that. Let’s see who gets pulled over.
I’ve been coasting with the clutch in to get lo rpm’s
After you do this for a while, I’d be interested in knowing if your clutch throw-out bearing begins to fail prematurely.
I was thinking the exact same thing. This practice could wind up being an example of the classic “Penny wise, dollar foolish”.
A classic “pay me now or pay me later” scenario. That little throw-out bearing is not designed for extended periods of rpm.