Re: overdrive
There's a difference in most vehicles between the tow/haul mode or trailer mode and overdrive. Overdrive is in most cases a 4th or 5th gear, tow/haul mode will change the shift points and pressures as well as often delete torque converter lock up from either overdrive or often all gears.
I would not use overdrive on most vehicles if towing a trailer that is more than half of its towing capacity, with or without a towing switch present.
The larger GM automatic trans, 4L80E and similar, need not much worry when towing a boat, and the same for the Ford 4L60E and E4OD transmissions, and the newer versions of that box as well.
What it boils down to is that if your vehicle is lugging in OD, shift to the next lower gear or turn off OD.
Newer vehicles such as GM which offer the Tow/Haul option handle larger loads much better in that using the Tow/Haul mode, it adapt the trans to a more aggressive shift patter as well as raises control pressures to make up for the added stress of towing.
If you don't have this mode, it's pretty much a judgment call, it's not a 100% fact not to use OD, and of course, not running in OD will hurt mileage, as will using the Tow/Haul mode.
Even more important on lighter vehicles, without the towing option or switch is to make sure you have a sufficient trans cooler for towing large loads.
Heat is the enemy of any auto trans, keep it cool and it will survive most anything within reason.
My Ranger has an OD manual trans, when towing any larger boats, about 1500lbs or more, I forgo using 5th gear for power reasons, as well as not running the AC as well. Doing so allows the truck to get the job done safely.
(Even without a trailer, I rarely use 5th gear unless on the open highway as its far too tall a ratio for much less than 65mph or better).
Much more important than not using OD, is to disable the lock up converter feature on most auto trans vehicles. Lock up in all but a few transmissions is achieved by a single friction clutch within the torque converter itself. It takes very little to burn this clutch under extreme loads.
Many transmissions will skip or disable converter lock up in manual third or if OD is not selected. The best option it to check your owners manual for proper towing procedures and to get a feel for the normal operation of your vehicle.
