I have a 1996 Honda Accord (automatic, did they make any other?), in great shape. I’m starting to adopt some of the “Hypermiling” techniques, and wanted to know: If I shift from drive into neutral and back, while the car is in motion, am I damaging my transmission in any way? Or any part of the car, for that matter.
Any damage to the drivetrain will be minimal. However, you won’t save any fuel. Your car uses less fuel coasting in gear than coasting in neutral. Coasting in gear, fuel delivery is shut off. Coasting in neutral, your engine must burn gas to keep running.
Ditto, and you’re losing some control of the car. Absolutely no benefit.
What? I don’t know of any engine management system that shuts off fuel to the engine when coasting in gear. If that happened, the engine would stall and you would have no power steering/brakes. You must have fuel going to the engine to keep it running so these systems work under all driving conditions.
Tester
Huh. I would have thought that since the RPMs are lower, the gas consumption would be lower.
Thanks for the info!
If the car has modern EFI and is in gear, wouldn’t the car’s momentum keep the engine turning? On some modern engines the valves even close when power isn’t needed. See Honda’s newest hybrid engine as an example.
Well, you would have brake boost, just think of the vacuum! Although…your ABS if you have it would not be functioning…
But no power steering. Just think of the excitement! Careening down a mountain pass with no power steering!
And think of the wonderful array of ECU codes!
True, but I would think the engine on a 96 would be too old to have that kind of thing.
Nope. The engine needs gas to keep running properly. It would be axactly the same as turning the key to “off” while going down hill while in gear. Without some combustion keeping the engine running, you’d be feeling purely vacuum, pulling the car to a stop. Your tires would hop (if you had a RWD), your fillings would fall out of your teeth, and your eyeballs would go “splat” on the inside of the windsheild.
If the vehicle is equipped with automatic transmission, the slippage in the torque converter will prevent the engine from rotating from vehicle momentum. With a manual transmission in gear, the engine will rotate with vehicle momentum, but a rotating engine isn’t producing power or vacuum. With no power, the power steering pump isn’t rotating fast enough and with no vacuum from the engine there’s no assist for the vacuum brake booster.
So no. The engine continues to run when coasting in gear. And this is controlled by the Idle Air circuit.
Tester
In the few days I’ve been trying this, I have yet to loose power steering or brakes while in neutral. I lost them the one time I tried going to neutral and turning the engine off (never again, even though that WOULD save me gas)
So the general consensus is coasting in neutral = no gas saved?
What absolutely baffles me, and I mean REALLY baffles me, is the all the trouble people will go through to try and save like 1/10 of a gallon of fuel over like 300 miles, when they could just get a car that gets decent mileage. “Hi I drive an SUV and my mileage jumped from 10 mpg to 10.4 mpg with all my crazy hypermiling techniques!” Arggh, good grief just go an buy a car that gets 30mpg! Now that’s what I call hypermiling.
“Oh but I can’t afford a new car” Why does it have to be new?? Just go find a car of equal value. Hell if you only have $500 go find an old beat up Honda CRX that gets 50mpg. I mean seriosuly, what is wrong with people?
Rant over, feel free to continue with this discussion 
Yup! Give it a while. And soon we’ll have some of these hyper-miler’s coming here and asking, “I’m a hyper-miler! How come brakes only last three months?”
Tester
I’m a long time hypermiler, since before the word “hypermiler” was coined. Why were the brakes on my car already worn out with only 160,000 miles on them?
My 2002 VW NB TDI does exactly that, as long as the engine is turning over above about 910 rpm no fuel is being delivered. It will start back delivering fuel below that to maintain idle speed. I understand a few gasoline cars do likewise.
He said he put it in neutral, not that he shut the engine off.
And a turning engine WILL produce vacuum, whether or not fuel is being delivered. But the engine won’t turn in a car with an automatic if the fuel is cut off, as has been already noted.
All in all, though, coasting in neutral is a fruitless idea, and shutting the engine off is just plain suicidal.
A rotating “shut off” engine isn’t producing vacuum? I would respectfully beg to disagree on this point.
As long as the intake valves are opening and the pistons being pulled down the cylinder, the engine is producing vacuum. Provide an obstruction to the airway like a closed throttle plate and the vacuum goes even higher…the piston is still trying to draw in the same volume as if the throttle were wide open. That’s purely a function of the displacement.
Truckers, have your “Jake Brakes” at the ready!
Brakes are the hypermilers worst enemy! They use them as little as possible (as do I).
So is it really true that you use less fuel in gear than nuetral (for a modern automatic car)? Even if this is the case, in gear you would get less DISTANCE than in nuetral, correct? Isn’t the distance really where you save on mileage?
If there is a difference it is so small that it is not worth the hassle. If you want instant results go 55mph and keep your tires inflated-that’s all there is to it.