I have a question. Last week when pulling our 35ft fiver I noticed the trans would not go into 6th gear until about 63 mph. If I remember correctly it used to shift into 6 at around 58 mph or so.
My question is can GM go in an program the shift speed like they can on the larger trucks?
I did change from the 16 in tires to the new 17 in tires and rims that are on the 08 models. would this make a difference? I can tell little difference in the speedometer calibrations.
thank for you thoughts I have to get it serviced this week so I wanted to get my Ideas straight first.

I would think it works on rpms not mph. When you changed to 17" wheels I'm assuming the tires are a larger diameter in which case you're turning less rpm for the same mph

The Tow/Haul mode changes the shift points (higher) hence when your in T/H it will hold lower gears longer. If you used the factory wheels and tires off of an '08, you didn't change the effective gear ratio any. GM downsized the tire section when they increased the wheel diameter. Both the 16's and the 17's stand about 31" tall. -Murph