1. It takes a little bit for it to get up to speed, especially if on a long hill on the highway, but 4th gear is an acceleration gear at highway speeds, even on hills, and the truck is happy to cruise at 75mph. It has an easier time maintaining 75mph in 5th gear on a slight hill than 55mph in 5th gear, though 55mph gives greater fuel economy in the long run.
2. I have maxed this truck out at 99mph twice - never breached 100mph. One of these times was coasting!
(Eisenhower - Silverthorne on a dry road)
3. On a normal highway (I-25 from Castle Rock to Denver) I can reach and maintain 90mph if I would like, much more than that and any loss in speed takes significant effort and time to regain.