I have recently been playing around with some hypermiling techniques here in rural western Oklahoma where it's wide open an no traffic for miles. On long downhill grades I am able to turn off the engine and coast for better than a mile with the auto trans selector in "neutral". I got to thinking that this may not be good for the automatic transmission. I know some automatic cars cannot be towed behind an RV without having the drive wheels on a dolly and this would be the same effect as flat towing. Do the short coasting distances make it a non-issue, or could this be potentially detrimental to the transmission. Buying a new transmission would negate the gas saved for sure!

 

Yes it will ruin the transmission. The transmission oil pump is driven directly by the engine via the torque converter, if that is not turning, you have no oil flow in the transmission.

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