Ok got my truck back today from the dealer. They replaced 2 O2 sensors and the NAC cat. Came straight home and hooked up to the trailer and took her down the hwy. Yaaaaah no more gear hunting and it stays in 6th except on a couple of the steeper, longer hills. It down shifts sooner then the old truck, but that I can say yes it may be the big tires..or it was the wind today (30-45 mph).... The fuel mileage is also where it should be that I can see so far, averaged out at 11.5 on that short trip, a darn sight better then the 8 it was getting.