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Old 12-22-2007, 01:13 PM   #1
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Autotrans coasting to stop

What I would like to test for mpg improvement in my car which is an automatic is: engine-on coasting to stop in neutral. Then, of course, back into drive after stop.

Will this harm the automatic transmission?

My manual says that for towing, the front wheels should be off the ground, rear-end up, or all wheels off-ground. I think this is a different situation...i.e., the engine is off.

I've tried it a few times and it certainly seems better than coasting to a stop in gear since the engine goes to idle-rpm's in neutral and there is no engine braking.
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Old 12-22-2007, 01:29 PM   #2
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Leave it in neutral after you stop...used less fuel than in drive at a stop. Shift to Drive when you need to pull forward. Of course bumping up tire psi helps before you coast ;-)

Jump in and start up a "Garage" space and keep track of mileage...takes a few fillups to get a real handle on mpg.
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Old 12-22-2007, 01:42 PM   #3
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Yes, I've gone from the recommended 32psi to 40psi. The tire itself says 44 max but I didn't want to push it.
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Old 12-22-2007, 04:57 PM   #4
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auto trans won't mind. stay in neutral as much as you can and bump the tire pressure to the max on the sidewall. I (and others) recommend against turning the engine off because without the engine spinning the torque converter, you won't have any pressure or lube in the spinning transmission internals
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Old 12-24-2007, 04:27 PM   #5
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Dropping into neutral at the right times can definitely give you a significant FE improvement. Pay attention to the lay of the land; around here there are a lot of gentle down-slopes that let me maintain my speed (or only drop it very slowly) in neutral for quite some distance, during which time I often maintain 150 to 200MPG.

Stopping can be a different matter. Since you have to shed all that kinetic energy anyway, neutral's not always the best answer, since DFCO can cut your fuel usage to zero during parts of an in-gear coast. Basically, you need to experiment and see what works best for your vehicle and regular driving conditions. It may be a combination of in-gear and neutral coasting.

Right now I'm experimenting with dropping into neutral as soon as a distant light goes yellow (to only use idle fuel while keeping most of my speed), then dropping back into gear and even down-shifting when closer to the light to take advantage of DFCO and slow for the stop. I suspect swapping those might be better (and improve the chances the light will cycle green again before I get there) but traffic considerations usually make that impossible for me, so I can't test it.

Being in neutral while idling saves fuel too, though not much in the grand scheme of things... still, all the little gains do add up. I save about 10% of my idle fuel with neutral versus being in gear.

Bottom line: Yes, informed use of neutral on an A/T can make a significant difference.

Whether it's potentially harmful to your transmission and/or torque converter is another matter. I still haven't been able to get a straight answer on that for my vehicle. You're right about the towing cautions being a different issue, however.

Also, be aware that rolling in neutral is technically illegal in some states or under some conditions.

Rick
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Old 12-24-2007, 08:02 PM   #6
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Thanks for the replies. I also read through many threads and found no clear assurance that neutral coasting won't harm the automatic transmission.

I'm too cautious to risk having an A/T repair (or replacement) which would I'm sure cost magnitudes more than any FE savings.

I think I'll stick with 'driving without brakes', anticipating lights, reasonable acceleration, higher tire pressure, and an even cruise throttle.
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Old 12-24-2007, 08:15 PM   #7
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to the best of my knowledge, the issues associated with flat towing an auto is overheating the tranny and fluid. See, when the engine isn't running, neither is the pump, so fluid isn't circulating. So when you flat tow an auto, the drive wheels are turning the tranny also. This causes heat and friction that would otherwise be addressed by the cooler. Over a short distance, its not that great a concern, but over a long distance it can destroy a tranny.

Coasting in neutral with the engine running shouldn't be an issue because the engine drives the pump in the tranny and is continually circulating fluid thru the cooler, and heat along with it. Double check on this but I'm pretty sure that is the reason why they advise not to flat tow an auto too far.
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Old 12-24-2007, 08:56 PM   #8
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Quote:
Originally Posted by VNelson View Post
Yes, I've gone from the recommended 32psi to 40psi. The tire itself says 44 max but I didn't want to push it.
Push away - you're really not pushing it at the marked sidewall pressure anyway Besides, the load rating for the tire is only good at the marked sidewall pressure A bunch of us pump up to infinity and beyond (well, 55+ anyway)



As for the tranny.... It should be fine. Problems arise if the lube pump disengages when in neutral - I highly doubt that's the case
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Old 12-25-2007, 09:21 AM   #9
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Usually the pump is only disengaged in park if at all. if you coast in park it may be an issue but there are more immediate problems then. coasting in neutral with the engine on is absolutely fine unless you have a freak of a transmission made out of glass.
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Old 12-25-2007, 10:54 AM   #10
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Quote:
Originally Posted by kamesama980 View Post
if you coast in park it may be an issue but there are more immediate problems then.
I definitely agree that coasting in park isn't something anyone wants to try.

Happy Holidays to all,
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