I drive 35 miles ea way on the 15 in SoCal, where typical speeds (non-rush hour) run 80-85 for the left two lanes. My 2016 Kia is happy doing that, and I have been in the habit of coasting down the long stretches, in neutral, of 4-5% grades many of which are over a mile. The speed will top out at 85 when I do that. Is this a problem for the transmission?
Coasting in neutral
- Thread starter srfdude
- Start date
I assume automatic.
When disengaged, no. When you put it back into gear, I'd assume not but I do not know.
A pal who was "neutchin' it" down a hill was told by the state policeman who stopped him that it is illegal to plummet downhill in neutral. He knew that was what my pal was doing by the signature speed (of that hill) of 74 mph.
I'd also assume that an automatic would have speed input and would know to be in highest gear when you pop it back into "D".
However, I never liked the haunting notion that there might be parts in the transmission which aren't moving when in neutral and instantly have to do 75-85 mph dance.
When disengaged, no. When you put it back into gear, I'd assume not but I do not know.
A pal who was "neutchin' it" down a hill was told by the state policeman who stopped him that it is illegal to plummet downhill in neutral. He knew that was what my pal was doing by the signature speed (of that hill) of 74 mph.
I'd also assume that an automatic would have speed input and would know to be in highest gear when you pop it back into "D".
However, I never liked the haunting notion that there might be parts in the transmission which aren't moving when in neutral and instantly have to do 75-85 mph dance.
Why? That’s a terribly dangerous practice as you’re losing a lot of your vehicles ability to control certain situations, engine braking, drive train input, braking control etc. It’s also illegal in many if not all states! Are you trying to save a thimble full of gas? 
Yes, it goes into D with no hesitation. I feather a bit of gas when shifting back to D, the engine speed matches smoothly.
I've been doing it for 40+years. Automatic and manual. There are pros and cons.
I always did it when I lived in IA AND NO other cars on the road. MT though. Ed
Why do it? The ECU has to feed the engine to keep it idling in neutral, where fuel is shut off if coasting in D with your foot off the gas. You're shooting yourself in the foot in a few ways, including mpg.
This.
Coasting in neutral is illegal in most states. You basically do not have full control of the car.
In practice you will use less fuel coasting in neutral with most modern cars. The fuel used by the idling engine is less than the energy gained by eliminating engine drag. There are other pros and cons. To each his own.
As stated before several times: Modern fuel injected cars completely SHUT OFF fuel delivery when coasting in gear. Going to neutral and forcing the engine to idle ADDS fuel to That downhill trip. You may add speed in neutral but you don’t save gas.
Been illegal in your state for 60+ years.
“The driver of a motor vehicle when traveling on down grade upon any highway shall not coast with the gears of such vehicle in neutral”.
Ca. Veh. Code § 21710
Enacted by Stats. 1959, Ch. 3.
“The driver of a motor vehicle when traveling on down grade upon any highway shall not coast with the gears of such vehicle in neutral”.
Ca. Veh. Code § 21710
Enacted by Stats. 1959, Ch. 3.
With a manual transmission, the engine continues to use a small amount of fuel (whatever is required to keep the engine idling) when coasting in N.
When coasting in gear, the fuel injectors actually shut off, and the car uses no fuel at all.
But even in an automatic, the difference in fuel consumption when coasting downhill in N vs D must be minimal, isn't it? A ScanGauge displays instantaneous fuel consumption, and fuel volume consumed per hour, and would let you know for sure.
When coasting in gear, the fuel injectors actually shut off, and the car uses no fuel at all.
But even in an automatic, the difference in fuel consumption when coasting downhill in N vs D must be minimal, isn't it? A ScanGauge displays instantaneous fuel consumption, and fuel volume consumed per hour, and would let you know for sure.
When driving an automatic, does your tach drop to idle when you're on the highway and take your foot off the gas? Nope.
Your road speed keeps the RPMs up, and the fuel injectors are shut off, until the transmission has downshifted to first gear and the engine eventually dips to idle, then the injectors return to pulsing. Drop it into neutral and lose that coasting energy, and the ECU must intervene to keep the engine running, through the use of fuel. Worse mpg than leaving it in gear.
Drop a manual transmission into neutral and release the gas, and your engine automatically returns to idle, where the injectors must pulse to keep it running. Worse mpg than leaving it in gear.
The engine will shut down if the ECU shuts off all gas to the engine. What happens, at least in BMWs, is the ECU (DME in BMWspeak) runs a very lean mixture, usually about 16.5:1 air/fuel mix. The optimal mix is 14.6:1 air/fuel mix by mass.
