Quick question about going down hills...
 

I have been searching around and can't seem to find the answer to this question.

When going down hills is it more efficient to leave the car in gear and engine brake, or put it in neutral and coast down?

I am not quite to the point where I feel comfortable turning the engine completely off so I want to figure out the most efficient way while leaving the engine on.

I remember when I used to have a wideband o2 sensor in my car the air fuel ratio would skyrocket when I was engine braking (it would read so lean that it would go off the chart). But when I was coasting in neutral the engine would idle around 14.7. So it seems to me that I would be using less fuel if I left the car in gear and rode the gear than if I put it in neutral.

I assume this has already been covered time and time again but I couldn't find a thread about it so if someone could post the link that would be excellent!

Thanks!

I haven't seen it covered and would like to know too.

It depends.

If you need to brake anyway, engine brake, engine braking uses 0 gas.

If you can gain lots of distance by coasting, coasting, engine off coast, at that, and if you need to slow down bump start and engine brake, :p

EDIT: The honda rule of thumb is above 1100rpms in gear with no throttle = 0 fuel injected.

90CivicStandard:

As SVOboy said in his edit, there are many Hondas that actually shut off the fuel injectors at throttle close in gear. So if you have to brake anyway, engine braking is really still not consuming fuel. But it is consuming momentum - so in a situation where you won't need to brake turning the engine off an coasting will be more effecient.

Also, as noted in another tread, you can run out of assist if you need to brake many times over the descent. It can be dangerous. Until you have the chance to "practice" on a "safe" descent with EOC, it would be wise to be less efficent until you do become comfortable with it.

yup as the others have said there is an rpm range where neutral is better and that depends on the engine. but for most purposes leaving it in top gear for decent down hill is the best alternative if you dont want to shut off your engine.

the reason for this is your iacv. it is totally shut while rpms are above a given value(1100 as mentioned by svoboy). when there is less air there is less gas even though the engine is spinning faster.

leaving it in gear is also better for the life of the engine for a couple reasons. the revs are higher so there is more oil pressure you wont ever see a difference but its one of those little things that helps over 200k miles. also when there is the most vacuum in the intake manifold(which is greatest with 0 throttle and while in gear) the pcv system does the most work. your lowest pressure in the crank case will be while you are in gear with 0 throttle aiding in a very small way with efficiency while aiding in removing some of the h20 that is hangin out in the crank case and would otherwise make your oil more acidic if it were to stay there for long periods of time.

sorry i dont have any links, its just stuff ive accumulated over the years. i know someone can pull up some links. if you look up what a pcv is supposed to do youll get some answers.

the life of the motor argument is pretty weak, but true. bottom line though, leaving it in gear saves more gas. sorry for the novel

:)

That's great info. I never knew the reasoning behind what goes on while decelerating in gear. It makes total sense that the more vacuum in the intake manifold, the more crank case vapors are being sucked out, which in turn would let the crank spin with more efficiency.

Do you guys have vacuum gauges that you monitor? I have been thinking for a while about picking up this autometer unit that we sell here at my work.

https://www.autometer.com/img/products/2337_d.jpg

It's very inexpensive and is already labeled for us gas savers!

I'm sposed to be getting one from a friend but I need to get on his case about it, :p

actually the vacuum helps the the crank side of the cylinders as there is less air to push out of the way.

As I've been thinking about this I've come up with another question.

If the injectors aren't squirting any fuel while going down hill in gear, then wouldn't that negate the need to turn the car off? If it uses zero fuel either way then why would you need to turn the car off? Why not just leave it in a high gear like 5th where there is very little resistance?

Loss of momentum. If you don't need to brake for even part of the descent, you are going to be more efficient if you aren't in gear.

That's not to say that there aren't downsides to it as noted above. But it will use less fuel overall.