You bring up an interesting point. The truck is quite light (as far as trucks go) and I know the rpm's would be way down in overdrive - 1900-2000. I thought that this would equate to much better gas mileage, and assumed that for most highway driving it would be able to stay in OD. On the prairies here in Calgary it's pretty flat.
Even if it kicks down a few times, isn't that exactly what I have right now with the TH350? How could that be worse?