Does coasting in Neutral Work

[1993CivicVX]

I'd say for all you auto trannies out there that, unless you live in an area that is really flat, to just use the hills as P&G and the rest of the time leave it in drive. The fuel savings are not going to be wroth the extra wear on the tranny going from D to N to D all the time.

As for why P&G works, think of it this way: The amount of fuel pumped into the engine is not linear to the amount of power produced. So if you feather foot the gas pedal, you are using more fuel per horsepower produced than if you press the gas pedal down hard enough to be in the engine's "sweet spot" for efficiency. In other words, at 75% or near full throttle (closer to the engine's ratio of fuel efficiency to power sweet spot), you are going to produce the most horsepower per unit of fuel used. I don't know exactly what this sweet spot is on most cars, but the point is, holding constant speed, while requires little power, isn't requiring *that much less gas to maintain*: it's an inefficient use of fuel for the amount of power it's producing. So when you are accelerating moderately you are only using about 2 1/2 times more fuel than when you are maintaining constant speed, but it's allowing you to spend much more time coasting, which uses about 5-10+ times less fuel than constant speed. That is why P&G becomes less effective in situations where the car is slowing down quickly during the glide. If the car is slowing down quickly during the glide, it means it is requiring more power to maintain constant speed, and therefor is closer to the power/efficiency sweet spot which apparently is somewhere above 60% full throttle (although I think it is less than full throttle) Anyone know where abouts the sweet spot is?

To conclude the argument for P&G....

So instead of driving at a constant speed that is inefficient for the amount of power it is producing, keep the engine in its most efficient power to fuel ratio for as little time as possible, and try to have the rest of the time driving in neutral!

[popimp]

It's not practical to do it for long trips! I got the same mpg using the engine on coasting technique at a 60mph average as I did with setting it at CC to 60 mph. I can say that speed does decrease mpg though. At a 70 mph cruise I only got between 24 & 25. So on this end I don't think it works. I'm gonna try and get together with Red and do some more testing.

[Danronian]

Has anyone tested this for the VX? I've been coasting down all hills in neutral lately, and it has netted me my best avg mpg (45-48) since owning the car, but I wonder if leaving it in 5th would be better...

For this car, the reason I don't resort to shutting it down, is because it takes most wideband sensors around 10 minutes to warm up, so I don't want to avoid being in lean-burn if I can help it.

[8307c4]

Only time I coast in neutral is when the engine is off, now that helps.

Usually before well known intersections, the light turns red, gear lever to N, engine off, coast to stop. It helps to be familiar with the light pattern, turn it on when the opposing light turns yellow (and mine is about to turn green), or if I have cars in front of me, by the time the 3rd car in front of me starts to move it is high time.

A properly maintained and well tuned engine helps as well, very embarrassing if it won't start now, preferrably it should start with just a 'kick' of the key, 1-2 revolutions at the most.

[1993CivicVX]

Originally Posted by Danronian

Has anyone tested this for the VX? I've been coasting down all hills in neutral lately, and it has netted me my best avg mpg (45-48) since owning the car, but I wonder if leaving it in 5th would be better...

For this car, the reason I don't resort to shutting it down, is because it takes most wideband sensors around 10 minutes to warm up, so I don't want to avoid being in lean-burn if I can help it.

But your o2 sensor isn't going to cool down in one or two minutes. If my CE light is any indication, it takes 5 minutes of the engine off for it to cool down.

[JanGeo]

Part of the problem is that the engine still is burning probably 0.5gph at idle so you do not save much fuel coasting then the added automatic tranny losses offset the savings. Best to keep a light foot on the pedal and keep the body surface clean. Think of it this way, if you turned off the engine for an hour you would save 0.5 gallons of fuel at idle so coasting for an hour would save you ??? . . . not much.

[Sigifrith]

Originally Posted by JanGeo

Part of the problem is that the engine still is burning probably 0.5gph at idle so you do not save much fuel coasting then the added automatic tranny losses offset the savings. Best to keep a light foot on the pedal and keep the body surface clean. Think of it this way, if you turned off the engine for an hour you would save 0.5 gallons of fuel at idle so coasting for an hour would save you ??? . . . not much.

Isn't the question how much I save in that hour of coasting VS what I'd burn at 55 MPH?

[Danronian]

Originally Posted by Sigifrith

Isn't the question how much I save in that hour of coasting VS what I'd burn at 55 MPH?

Yeah, that's what I'm wondering. Here's an example of what I'm wondering:

Going down a long hill, where no throttle is needed (a few minutes of coasting), would it be better to be in 5th gear, which would have the motor at about 1500 RPMs or would it be better to be in neutral at 600 RPM?

For the VX, I'm wondering if idling wastes more gas than being in lean-burn?

As soon as this tank is over (and I get a base-line for my car with my shift light now functioning and following those points), I'm going to stop coasting in neutral and keep it in 5th instead. If there is a difference in MPG, I would assume the VX would prefer to be in gear going down hills.