In my experience, overdrive produces better MPG when the transmission stays in overdrive. In other words, it doesn't shift in and out of OD, in the manner Wayne describes. But if you start going up and down small hills, and the tranny starts shifting in and out of OD ("hunting"), the MPG advantage disappears, and the tranny temp goes up. If it gets out of hand - and you don't know what's happening if you don't have a gauge - think future expensive damage.
The transmission control computer is programmed to get best mileage out of the vehicle alone, when it is not towing anything. In most cases, it doesn't know how to handle the weight of a trailer, and keeps pretending that the vehicle is unloaded. In other words, when you add a towed weight (the TM, in this case), the computer control no longer optimized. Vehicles with a "tow/haul mode" are a different story, and this is one of my few gripes with my Explorer.
The price of a few extra gallons of gas is trivial compared to the price of a transmission rebuild.
Bill
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