Tire sizing for towing

[BVP]

I have a 2007 6.0L F250, it came with 265/70/17s and when they wore out I moved up to 285/75/17s, about two inches taller. I noticed I lost a little on the lower end while towing and a few mpgs. Now I need new tires again, and I'm thinking about going back to 265/70/17s or maybe even a step down to 245/75/17. Is there a problem with running the dually sized tire on a SRW truck? Do you think I'll notice a real difference going from a 34" to a 31.5" tire in power/pulling or mpgs? The truck only runs on the highway and is usually towing. I'm just starting to not be as satisfied with how it's pulling, and was wondering if dropping tire sizes would be better than going straight down the tuning road. Smaller tires + tuning = even better?

[F350Powerstroker]

New gears = even better...

[JasonY]

Stay at or under 33" and you should be fine. How much are you pulling? Seems like 33's are the cut off for really effecting your effective final drive. These trucks with 34's dont tow worth a darn imo stock. Even worse with my 35's.

Jason