I'm really surprised by how many people are saying tires and especially tire size don't affect gas mileage. In my experience, moving up in size can dramatically affect highway mpg. Especially when combined with a lift - even if just a leveling kit.

 

It takes more energy to rotate a larger and heavier wheel, and it can definitely be noticeable in MPG's

 

He didn't ask about a lift. And he didn't ask about monster-truck tires. He's asking about the difference between 33" & 34" tires, which is NOT significant.

 

Well, as mentioned, he does have a lift on the truck. And if he's going from the stock 275/55/r20s (could be mistaken?), and has been able to get 20 mpg hwy out of that setup, he will be hard pressed to get 18 mpg moving up to either of the sizes mentioned. That is a noticeable difference.