Im running my 700R4 with 3.73 and 35" BFG Mt's. Of course, my bfg's are closer to 33.5" true height on the ground. Anyways, it all depends. I acutally like it eith 3.73's, and on my 600+ mile trek back and forth to NC, I got around 15-16mpg on the highway. I do a lot of highway cruising at around 70mph, its my perfect gear ratio.
Oh yeah, I didnt realize how much heat my tranny produced, until I put in a stacked plate cooler. I kept the old tube finned one as well, as well as the one through the radiator. But ever since I've put it in, my tranny fluid looks, smells, and even tastes (just kidding) like new.