Overdrive on or off?

I'm a retired truck mechanic, most over the road trucks have a transmission temp gauge. Reason being heat is one of the things that kills automatic transmissions. If the transmission is shifting frequently then it is being overworked and will heat up. The heat breaks down the fluid and that in turn causes excessive wear. Changing the AFT fluid every 30k miles will prolong the transmission life considerably. My personal belief is if you can run in OD and hold that gear you should be fine, I do know there are two schools of thought on this issue and I have technicians argue it back and forth.

If you had a car with a 5-speed stick shift, would you never use 5th speed? What you're doing is the same, IMO.

frankdamp wrote:
If you had a car with a 5-speed stick shift, would you never use 5th speed? What you're doing is the same, IMO.

Absolutely, if you couldn't pull 5th gear or kept downshifting around every corner.
However, that's the main advantage of a manual trans, being able to pull high gear longer without damaging the transmission.
With a stick you can pull any gear you want until the engine won't pull it (not talking about lugging a diesel and egts here) and you'll do no transmission damage. With an auto trans pretty much regardless of design, if you're not able to keep the converter locked and/or downshifting a bunch you're making unnecessary trans killing heat.

Generally automatic transmissions are programmed to shift based on load and RPM, the engineers that develop these shift points know what there doing, what they don't know it the environment your engine is running in. One thing I do know for a fact is if a transmission is shifting constantly it is building heat, and wear and that is not a good thing. For me I want my transmission to keep my engine close to it's peak torque curve, that is going to put the least amount of strain on the engine and produce close to the best fuel mileage.
A standard transmission normally doesn't care how many times it is shifted as long as the driver isn't banging gears. Some drivers do have a tendency to lug an engine that is backed by a standard trans.