Running of car in neutral gear

Maybe in the old days, but modern cars have computer controlled fuel injection which should take into account the load on the engine and only inject the amount of fuel needed.
Del

I don't believe Del gave a completely correct answer. While it may be true that the computer reduces fuel consumption based on engine load, with the car in gear, the engine acts as a brake and reduces the speed that would otherwise be obtained rolling downhill in neutral. There is no question you get better gas mileage rolling downhill in neutral with the engine idling, compared to in gear.

However, this is not the only issue. The engine acting as a brake controls your speed better, and in the event of a sudden need to brake, you would reduce speed much faster with the engine driving the transmission. I told all my kids to never shift into neutral going downhill.

That being said, and being a Dad, I told them to do what I say, not what I do. Faced with a steep enough hill to maintain desired speed, I always shift my manual transmission automobiles into neutral and coast down the hill. My kids all drive automatics and it is much harder to shift into Neutral and then back into Drive while moving forward, making sure you hit Neutral from Drive and DON'T accidentally go past a little into Reverse.

Most of my cars are old enough to verge on antiques, but my newest is a 1999 model year with a computer and it is fun to watch the fuel mileage climb towards 99 mpg and peg out coasting down a long hill.

emc_c

Some good points...it prob depends what gear you are in when driving a manual.
IF the roadspeed is faster than would be obtained at idling speed in the selected gear then the engine management system should be able to detect that effectively the is engine braking and should thus shut off the fuel.
Maybe if you roll down the hill backwards the engine will actually generate petrol?
Del

And to answer the silly question that sometimes comes next...it is a bad idea to shut down the engine and coast downhill without power. Your power brakes will only work once then they become manual (without the leverage advatage of normal manual brakes) plus, your steering could lock or become difficult to steer without power steering (again without the mechanical advantage).

Best way to save gas, drive conservatively.

Drew

I REALLY hope Drew K is confused, and you never intend to turn the engine off to conserve fuel. That's very dangerous, as most modern vehicles have power steering and brakes.

And, "Drew K is correct about brakes, except the only work once, then they work at a greatly reduced level with engine off.

If you are suggesting just shifting into neutral and leaving the engine running, why bother.

Just pretend there's a raw egg between your foot and the accelerator and you'll get good mileage.

Right, that is what I meant. I am on holiday and only online on my iPod (sry for confusing posts) Drew

Right, this is a question that can only be answered if you know where you want to end.

If at the end of the hill you will need to slow down then you should let your engine brake for you. The injection system will cut off all fuel and you will save your brakes. If however you wish to and can re-utilise the momentum retained while rolling down the hill then roll. The fuel injection will supply enough fuel to allow the engine to idle and power the steering and brakes etc. and you will still have retained the energy supplied by the hill.

Simples!

ps. If your car has a carburettor or mechanical fuel injection disregard all the above and switch off all the time

Fuel flow to a car motor is very slow, and difficult to measure. A computer controlled fuel injection system might be able to be monitored accurately. It would be so cool to watch your fuel consumption at any time. I've been told that the computer dumps fuel into the engine to keep the catalytic converter hot when the engine is decelerating. Apparently, if the cat cools down, it won't work right.

Isn't that special.

Thanks to all.