I shamelessly borrow this from a different thread:
"When towing a heavy load, never use 4th (OD) -- keep the button OD-lockout "on" to keep it in third, and keep your car's speed so that the revs stay in the power band of about 3,000rpm to avoid overheating the transmission and/or engine."

It is not the first time I have seen this mentioned, and I have been wondering about this for a while. For the first time in my life I now have an automatic trannied car after almost exclusively stick shift driving for 30 years. Studies of exploded drawings have educated me in the inner mechanics of the automatic transmission, with its three "normal" gears plus the planetary gear.

What I would have liked to know though, is why this design; why not simply use four ordinary gears?

Also, is the O/D in any way "weaker" than the three lower gears as it should be avoided when towing? Or is the quoted advice exclusively related to transmission cooling: Using the third gear when towing causes less strain and consequently less heat than engaging the O/D? I see the point in not having the transmission changing up and down in gears all the time, but the rest of it is less obvious to me.

Thanks as always,
Erling.
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