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Old 03-26-2012, 10:28 PM   #10 (permalink)
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Originally Posted by hendgeofdeath View Post
I would like to start dropping my automatic transmission into neutral to coast to stop signs. Problem is I'm not sure if its safe if I have to go back to drive while still in motion. I have an 07 Malibu, and its not worth it to me if I'm doing any sort of extra wear and tear on the trans. Your expertise is appreciated.
What you describe should cause no harm, but here's what you need to know. The potential problem comes not with putting the transmission into neutral and coasting, but rather when you re-engage it into drive. If you coast to a stop (or an almost stop, meaning below 10 MPH) there will be no harm incurred.

Most automatics are designed with the intent of being shifted from neutral or park into drive, as starting from a standstill. The potential problem arises when you are moving at speeds much greater than that 10 MPH threshold of first gear. I once asked the same question of a transmission technician and he told me the torque converter cannot determine how to handle the sudden transition from neutral into a much higher gear. In other words you not only risk damaging the transmission but the torque converter as well. However if you coast to a stop (or an almost stop) before re-engaging back into drive, there should be no problem regardless of the transmission design.

I've been doing this for a long while now on my four cars (all are automatics) and have found no problem from doing it. However, bear in mind that if the stop sign that is your target point is 1/4 mile off and someone pulls out at a slower speed in front of you while you are going 40 MPH, you probably will need to coast back down to 10 MPH before re-engaging into drive. Thus I never coast to traffic lights - because if the red light turns green while you are approaching it at 30 in neutral, your options are few because you can neither go back into drive nor accelerate.

In my experience neutral coasting an automatic does help improve FE somewhat - but the situations where you can implement it are limited, as traffic often makes it impossible to do.
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