schu

I don't doubt that a long flat trip at highway speeds from station to station shows better milage with the 315s, but I doubt you are getting any better milage for any amount of combined driving for the following reasons:

315s pros to gas milage:
Lower highway rpm

315s cons to gas milage:
Gears spaced farther apart makes for more low end lugging.
Taller gear when starting and stopping (2nd gear is much taller off the line)
Much more weight to get rolling. The BFG AT is 64lbs a tire.

Another thing to consider is that in the 6 speed, the 315s bring down the rpms to about 1800 rpm at 65 which pulls the empty truck up a good hill fine (though it does work the truck a little more according to the EGT), but it won't tow 6k up a decent hill without going back to 5th gear which is 2200-2300 at 65. So if your doing any amount of towing your probably going to get worse milage unless it's all flat and the truck doesn't have to work to keep things going at 1800rpm in 6th.

I guess my point is that taller gears are better on long flat roads, but quickly become a wash when you put your truck to work.

I really don't want to go all the way back to a 265 so next time I'm going to get some 285s, my problem is that nobody really makes a load range D or E in that size except for BFG and I was hoping to try something different this time. Any one have a suggestion for a 285/70T17 tire that is rated for over 3000lbs?