I tow the speed limit +-5mph. Never a blowout or even a flat tire in over 60K miles. I've seen 350/3500 duallys pulling small TTs experiencing enormous amounts of sway even though they have a WD/SC hitch and 1/2 tons pulling 30+ foot rigs that are riding level and exhibit minimal sway. All I worry about is my set-up and it is rock solid when towing in high winds or passing commercial diesel trucks with little or no sway to speak of. Going a few miles above the speed rating of a tire will rarely cause catastrophic failure, under-inflation and overloading are the real culprits. As for mpg, I wouldn't be RVing if I was concerned about the cost of fuel, you have to pay if you want to play.