Now this is just my experience, but I have yet to see bigger tires get better mpgs. Not once. MPGs is also one of those tricky calculations that it's way too individual. In other words, you can have 10 people say that they saw an improvement with bigger tires(which I can't reconcile in my mind how that would be true and I've heard the bigger reasons why and a problem is that people tend not to do just the tires, they do something else that could have helped with with mpg as well, alot of variables) and yet you still may not, because factors may not be the same. I have yet to hit the average mpg that the vast majority of people with DRWs claim when I was stock or modified. That's one of the reasons why I tend to put very little faith in mpg claims.

If you are really concerned with mpg in these big behemoths then just slow down. That is the simplest and cheapest thing to do and it would atleast the same improvement that you would have seen with bigger tires(which like I said, I'm still iffy on). You might have to leave a little bit earlier to get somewhere, but you'll get your improvement as long as you don't have some quirky driving habits.