BlueRam2500,
I have the 48-RE and 3. 73 gears AND am running 315 BFGs (34. 6" diameter).
I am getting between 15. 5 and 19. 8 hwy mpgs depending on driving conditions, speed, wind (as klenger noted above, wind effects are more noticeable on mpgs), and throttle input (*grin*). In the city, you will better my mpgs, as the 315s and 3. 73 in stop and go driving hurt me some.
Freeway, I will better your mpg simply from rpm advantages. I think the 35s will help with the rpms at freeway speeds, but you will still need to keep your speed down (60-65 mph?) if you really want to see good mpg figures.
And again, I do know that the 315s I have are hurting my mpg ratings some, but I need the height/tread pattern for some of the places I use my truck... I just keep 'em pumped up (60-65 psi) on the highway, and they do what they are gonna do...
Good luck!