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Old 06-28-2008, 12:11 AM   #1 (permalink)
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Slow Speed Issues and an Auto Tranny

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!

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