6.6 Chevy gas engine

First question to 6.6 gas owners, do the 2020 gas HD's run 265 or 275 70/18's as OEM? Are you guys running OEM stock tire sizes? My 18 Duramax LB CC SRW 3500 gets 17-18 up to 70 mph. Over 80 it drops to about 15/16, running 275's now on that truck. I just pulled off the 285's which was an expensive test. Horrible mpg, lack of power, looked good, but not worth the offset in performance. My 20 DRW gets the same mileage as my SRW. I am running 245's instead of the OEM 235's on that DRW. I think this can be attributed to the 10 speed, the 20 is much heavier with a steel flatbed, 100 gallon offroad fuel tank and a steel storage box over the fuel tank and of course running an extra set of tires. Yet the MPG is nearly identical. I think if they would just drop the 10 speed in the 6.6 gas you would see at least 1-2 mpg gain empty and probably even better performance towing. I don't know what they are waiting for, using up 6 speeds sitting in a warehouse somewhere?