I have the 4.3 as well. I am running 305/50/20 Nitto Terra Grapplers on a 20x10 XD Wheel. Not really a taller tire as much as wider. My gas mileage hasn't dropped too bad. I can get 19 to 20 on the highway. The big thing I noticed is it seems to downshift a lot more. Especially on the steeper inclines on the highway. What would be the best route to take to keep this from happening? Regearing or maybe just a tune to give it a little more power?
My truck is currently sitting at the dealership getting warranty work done and has this same problem. They are going to put a new weather stripping on there to see if it helps. I will get it back Tuesday or Wednesday and let you know if it helps.