Use the overdrive.
It is designed to disengage if it puts an excessive load on the engine, and it will save fuel.
The only time you may want to disengage it is if going down long hills, so as to use the engine to slow down and save brakes. Like shifting from 5th gear to forth, same thing, or if you need additional acceleration.
Use it in city and highway, and don't be concerned about it.
Friday, February 13th, 2009 AT 7:09 AM