First of all i have 4:10 gears in my truck i am running a 265/75/16, gas mileage is poor, tuned, intake, exhaust, ect. Will a bigger tire help to drop rmps of motor to increase gas mileage, i apologise if this sounds stupid

 

with a bigger tire you will drop RPMs and on the highway in theory give you better gas mileage. but with a bigger tire also comes more weight so that acts agents that theory. around tow driving the larger tire will hurt you, give you more weight and less torque being applied to the ground to have the truck start moving.

i hope this helps out

i have 4.10's in my 04 2500hd with 285/75/16's and an intake and exhaust. with 87 octane i get around 11-12 highway/city, but with 89 for some reason i get 14 city and almost 16 highway. also i don't cruise at 65 unless towing a trailer so it could be better