Tires: Tire size, tire diameter, gas mileage

Question
I have a 2004 GMC K2500HD - 6.0 V8 w/ a 4.10 rear. Tires are 245/75/16 - want to change to 295/75/16. The purpose is to improve gas mileage. The increase would 9.7%. Revs/mile would go from 662 to 603. The speedo would read 60 but the actual would be 65. I would have it recalibrated. Would this adversly effect the drive train? Would this result in increased gas mileage?

Answer
Kent,

I hate to throw cold water on your idea, but the stock rim is too narrow for a 295/75R16.  If you have steel wheel, then the LT245/75R16's are the largest that will fit.

Even at that, changing the tire diameter is effectively changing the final drive ratio.  This would probably have implications for the warranty, but IMHO shouldn't really affect the drivetrain durability - and may or may not affect fuel economy.  There are just too many variables - and one of them is how you drive the truck.