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Originally Posted by Mike Donofrio
Ray, this is by far the best and clearest explanation I have seen regarding this oft asked question.
I generally leave the OD off when towing, although if I am on a long smooth and flat stretch of highway and I have something small being towed I might turn it on for a while to save a few RPMs and gas...
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Thanks, Mike
Yeah, after reading about a gazillion discussions here and on various trailer towing forums, I gradually concluded that hard and fast rules (e.g. "always tow with OD off") just aren't a good answer.
For one thing, transmission designs vary widely between manufacturers and sometimes between models or even years of a model for a given manufacture. Even for we Tundra owners, we with the '03 and newer models with a beefed up transmission probably can more safely tow in OD than those of you with the earlier model transmissions.
But another factor that has to be considered...especially by those of us who tow on Colorado's high altitude mountain roads...is the greatly reduced power we have available. At 11,000 feet (a very common elevation of mountain passes in Colorado), the Tundra V8 is only putting out about 110 HP. Even at a "low" Colorado elevation like mile-high Denver, the engine is only making about 160 HP (30% less than sea-level). With so little power available, it takes almost no grade at all when towing to necessitate enough throttle movement that a TC unlock or downshift will happen if OD is left on. Even though I'm towing a moderate weight, low profile trailer, my truck will only keep a TC lock in OD on a downgrade ...just maintaining 65 mph on level ground causes the TC to unlock in OD. With OD off though, the TC locks and stays locked even on some pretty good grades.
And finally, trailer weight and profile (aerodynamic resistance) has a lot to do with whether or not OD towing is wise. A low profile, light weight trailer presents much less of a load on the drivetrain than a big box, slab front, 5000 lb travel trailer.
Too many variables for hard and fast rules, IMHO. So I think that watching the TC's behavior is the only good clue we have. My recently installed,
pan mounted tranny temp gauge (so I'm seeing the temp of the ATF as it goes into the transmission's innards) has been a very useful confirmation of what I suspected was happening in the transmission.