danxp

i occasionally 'hypermile' by putting the car into neutral when going downhill...

is this bad for the car or transmission in any way?

bagwell

just puts wear and tear on the linkage....I do the same thing....trying to stop this practice...but I love getting 40mpg from my mazda 3

danxp

i always suspected it would be bad for the tranny if i put the car into drive while coasting at 65mph... but i wasn't satisfied with my crappy 19mpg on my ls430.

lobuxracer

iTrader: (2)

Depends. Lexus/Toyota/Scion all have a fuel cut over a small rpm range where the engine typically runs while in drive going downhill with your foot off the gas. So the engine is using even less fuel than idling. They do it for emissions control, but it has the side benefit of improving fuel economy. They've been doing it since at least 1988.

AFA damaging anything - pretty unlikely. As long as the transmission's input shaft is rotating, your gearbox (manual or automatic) will be getting enough lubrication, so there's really nothing to worry about in the long run.

Threxx

My 335i's torque converter is very aggressive and puts a lot of drag on the drive train... even in the highest overdrive (6th) gear letting off the gas feels like on a manual transmission car when you downshift a gear or two to slow yourself down... the car just doesn't coast efficiently at all. I understand this is intentional to make the car feel sportier but it really kills me ability to coast to red lights and down hills to get better mileage, so I constantly am putting the car in neutral. Of course it's a little additional wear and tear but I don't think it'd ever end up breaking anything. One think I try to make sure of is that I don't accelerate for a solid second or two after placing the car back in drive, just so the trans get re-situate itself.

This is one of the several reasons I like driving manual transmission vehicles better... in my Audi I was constantly in neutral... basically any time my foot wasn't going to need to touch the gas or brake for a few seconds, I'd just push the clutch in and take it out of gear... that Audi would coast seemingly for a mile or more on flat road if you put it in neutral at 50-60mph and just let it go.

SLegacy99

Ah, I've seen people do this with their automatic transmissions. Nothing good will come of it.

I, myself am one of those people that always drives like this. Probably the reason why I don't mind driving stick in traffic. I'd rather have the gas mileage...and the fun.

Faymester

I can recal a personal connection story where throwing the vehicle from neutral to drive often while the car was moving eventually caused the transmission to explode.

shyguy16

manual? no problem at all.
auto... no clue.

herbvdh

I did this once years back and when I shifted back to drive I heard a very loud backfire scared the $$$$ out of me.

mmarshall

But how about the output shaft, which simply spins wth the drive shaft?

And, automatics also have the transmission cooling tubes running through the engine's radiator. If the engine is in neutral, it may cool to the point, coasting downhill, where the fan shuts off and the radiator doesn't do much cooling except for what air flows through it from coasting. Although not likely except on a very long hill, that could (?) allow some transmission heat to build up.

There is also, of course, the BIG problem with coasting downhill....loss of engine/compressional braking in the lower gears, and what could be a huge load on the brakes if you have to stop quickly, possibly causing the rotors to warp.

-J-P-L-

As lobuxracer touched upon,

Many, if not most cars today, are equipped with DFCO (deceleration fuel cutoff) technology. It works whenever you decelerate with you foot completely off the gas and in gear (NOT in neutral). So don't throw it in neutral thinking you're saving gas, you're actually using more than if you just leave it in drive.

Most people are unaware of this tech which can save a decent amount of fuel if you consciously approach stops and traffic coasting rather than gassing it then braking hard (which many people foolishly do).

dunnojack

i just take my foot off the gas and let the rpms drop below 1000.
i still consider it hypermiling.

i may not be saving as much gas as in neutral??, but any gas savings is fine by me.

mmarshall

AWD is also a big benefit, going downhill, because of the added drag it placers on the drivetrain. I've noticed, with my Outback, that I rarely have to downshift or use brakes going down reasonable or moderate grades to keep speed under control.....only when the grade is very steep.

lobuxracer

iTrader: (2)

In an automatic the input shaft runs the oil pump, so ATF is flowing throught the gearbox. In a manual, if the input shaft is turning, so is the countershaft, and the countershaft is pulling oil up from the bottom to lubricate the output shaft. No worries. Life is good in either case.

If the fan shuts off it's because the thermal switch in the radiator says the coolant is plenty cool enough without needing any more assistance from the fan (which is useless over 35 mph anyway). This is why manufacturers developed mechanical fans with fluid couplings - so they wouldn't drive the fan and waste horsepower at highway cruising speeds. If the gearbox develops heat (but how could that happen if it is not being loaded?) then it will add that heat to the radiator, and the thermal switch on the radiator will kick on the electric fans (if the fans are electric) or, if the car is moving over 35 mph, the fan will be disengaged anyway because the silicone fluid in the fan clutch will allow it to freewheel (unless the fluid gets hot, then the fan will engage again). I know it's hard to believe, but we've come a long way since the 50's in understanding how to manage heat in modern automobiles. Heat management is directly related to efficiency, and despite the pundits, the engineers are always looking for ways to be more efficient so the company can meet CAFE standards.

IS-SV

Yes and since the topic is hypermiling/gas mileage we might as well mention the fuel economy penalty assoicated with that extra drag.

btw, I've noticed modern high-compression engines seem to have a bit more engine braking too, but maybe some of that effect is due to the modern auto tranny.