Slow Speed Issues and an Auto Tranny

[Bobsterz]

Care to offer a reality check?

Warning: Those of you with modern vehicles will laugh at this dilemma.

I was wondering when, at low speeds, I should pulse instead of coasting. I understand that traditional ICE's with injectors consume very little fuel when coasting, except that there is a speed at which they use more because it's more like an idling condition. True?

How about this rough calculation for a medium/large non-hybrid:

If the engine gets 1 gallon per hour (gph) when idling in Drive...
and I'm traveling at 10 mph because the engine is pushing me in idle...
then I'm only getting about 10 mpg.
If I'm going 5 mph, then I'm getting about 5 mpg.

I'm right, so far, right?

*But* if I pulse and glide in this slow speed situation between 15 and 25, maybe I'd be getting something like 22 or maybe 30 because of pulse and glide? (It rates 18 mpg city, 22 hwy, and this hypothetical is about what's happening without stops, so it should be better than 22, unless the slow speeds are more of a factor than the reduction in wind drag is.)

But since coasting much below 15 mpg appears to be detrimental, that would be a good time to go into Neutral, if you'd be coasting or stopped for very long. That's because gph is about half in Neutral what it is in Drive at very low speeds, if I'm understanding things. If I were coasting in idle on a little decline at 10 mph, I'd be getting about 20 mpg in that case.

TAKE HOME MESSAGE: If I'm roughly correct, if you can't pulse and glide from 15 mpg and up, then you should emphasize strategies that would promote being in Neutral, if possible. (The exact speed will vary with the vehicle.)

With that in mind... Seems to me that, if you're trying to "time" a light, you'd be best off considering the costs of slow speed in gear driving vs. the value of conserving momentum by driving really slowly on your way to the light. You have a little less flexibility in Neutral as to when you reach the light, but *sometimes* you'd be better off coasting in Neutral from a little higher speed, and risking having to stop for the light. I guess it would depend on how close you are to the light and how predictable the condition is.

Also, doesn't the best way to manage this scenario depend on when the engine adjusts itself from a coasting to an idling condition as it slows down? I don't know exactly when that happens and how much difference it makes. Can anyone chime in? How much fuel is used at higher speed coasting in gear? That would influence the "break point" on these decisions.

I can't use a scan gauge because I have a '91 vehicle. It's a Mazda mini van I bought so the rest of you could draft me.

And how much wear does going in and out of idle put on an auto tranny? I'm sure it's a lot less at slow or no speeds, so should I not worry about going in and out of idle in slow traffic conditions?

BY THE WAY: The ultimate measure of fuel conservation is not miles per gallon, but gallons per month. But you knew that!

[trikkonceptz]

First, if your car is burning 1gph at idle, you have issues to start with. My 4cyl Vibe at idle burns .15 gph, in order to get up to 1gph, I need to be cruising @ 55mph @ 2200 rpm with a LOD of 29.

Next, I don't how you can drive 15-25mph anywhere in the US unless you live in Omish country and only have to contend with horse drawn buggies.

With that said, my auto seems to like the speed range of 35-40mph where is sees mpg's of close to 80mpg.

Best suggestions to disprove your own theories is obtain one of the non OBT tools being developed/sold here and tweak your own vehicle.

[Bobsterz]

I just snarfed the 1 gph in Drive figure from a couple posts, but they were for 1.6 to 2 liter engines, as I recall.

And the slow speed is for those slow speed conditions you get into such as parking lots of various sizes and heavy but, moving traffic. I'm in San Diego and there are spots where it gets slow, but you can see well enough ahead to make the kind of choices I'm posting about. I'm thinking especially of some spots in the Pacific Beach and Mission Bay areas that I'm in a lot. Rumor has it that it's full of closeted Urban Amish surf bums. (A dying breed, due to iPhones and microbrews.)

I want to refine my sense of the ideal speeds to shoot for in those conditions, and the ideal times to be in Neutral.

Thanks for the tip about the non-OBT tools. I'm all for disproving or refining my own theories.

[RH77]

First, Welcome to EM...

The assumption of 1.0GPH may be a little high, which may effect the cascade of theories. Re-run the calcs at 0.5GPH and you may find a better result for coasting (20 mph becomes 40 MPG, 10=20, 5=10). Or 9999 mpg with the engine off

<Disregard that last statement, unless you are aware of the risks, practiced on a closed course with a professional driver, etc...>

Someone else can help with P&G, but the slow-to-light in neutral seems to be a tried and true method for a variety of vehicles. If I come in too hot, I downshift to force the injectors into "DFCO". The worst scenario is to start from a standing stop.

Another method (not as popular) is to keep up the speed and stop quickly. Your theories would apply here. 100 mpg, 100 mpg, 100 mpg, STOP!

RH77

[brucepick]

We should make this point re. auto tranny:
(In case anyone here's considering coasting with engine off)

Auto trannies usually have restrictions on coasting with engine off.
That's due to the tranny's input end driving it's lubrication system.

Check your owner's manual for flat towing. If the car can be towed with all four wheels on the ground at any speed then you can coast with engine off. Often it will say, for example, up to 20 mph and 20 miles distance. My old Volvo, 40 mph and 40 miles. Saturns often have no restriction; you see them towed behind RVs. Some years Saturn require that you remove a couple fuses before flat towing.