My new 04 LLY is running 265/75/16 tires. On the trip home the miles to empty were way high and the MPG were showing 23. That was with me running 75-80. Im guessing my computer is off because of the one size bigger tires. What do I need to adjust for the bigger tires? I hate to have to buy a programmer b/c I plan to leave the truck stock (except for egr plate and maybe pcv reroute).

have the dealer program with the correct size then.. may cost you $100...otherwise get a programmer, change size, gain some mieleage in the process.