With the very high cost of gas you are starting to see lots of mention of hypermiling to increase gas mileage and a big part of that, other then don’t get yourself in positions where you need to use your brakes, is to coast down hills.

My question: If you will damage an automatic transmission by towing it at high speeds in neutral (most automatics) will you damage your car by placing in out of gear and coasting down long hills at 65-70mph or does the transmission cooling lines take care of the damage problem because the engine is running?

PS. I tried just a little bit of this, decreased my speed from 70-72 to 65-66mph on my last 200 mile Interstate Hwy trip, coasted maybe a total of 2-3 miles in short burst on long hills and even with the A/C on and the trunk loaded with gear I got 32.8 mpg with my 2002 2.7 V6.

 

Well, if the engine is running, you don't have to worry about lube damage (seals). The only other thing is driveline slack or backlash. If you were to do this regularly it could turn into a vibration every time there is no torque on the driveline while coasting. This is especially hard on manual tranny's clutches. Mine actually downshifts on hills to keep the speed down while on cruise control. I usually tap the brake to shut off the cruise before going down a long hill.
It's good to keep a little torque on the driveline all the time.

 

Thanks for your reply Cordoba Man,
As stated above, I only did this a few times over a 200 mile trip but I now have a growling sound or vibration that is getting louder. It starts when I get up to 20mph and it only stops when I take a right hand curve or turn right. I thought maybe it was the powerstearing going out but was comfused about it only doing this past 20mph and stopping on a turn instead of getting louder. Now I suspect it is much more and will take it to a dealer. That little bit of extra mileage I gained on this trip may have cost me hundreds or thousands in service department charges???
I will update after I bite the bullet, *pay the Chrysler dealer service department price {{shudder}}*[:@] and get it fixed.
Beware, Positive Thinking ahead > I guess there is always the chance this noise is just coincidental, or it is just tummy growl because I tried to save gas and starved it! [sm=exactly.gif]

 

I think you'll find that you've got a wheel bearing or cv joint wearing severely. I doubt that coasting in neutral would be a primary cause for it. It may have hurried the impending failure though. Of course, this assumes that you aren't slamming power to it but easing into the speed when you coast.

 

Is this really worth the gas it saves for the effort and hassle?

 

My 2006 gets over 30 mpg at highway speeds without any of the hypermiller tricks.