The difference could just be in the calibration that CC did when they built it.

When I replaced the speedo in the other bus it had to be calibrated. The calbration value Prevost gave was quite a bit wrong, so on my first trip I think it indicated 20 or 25 mph at 55. I knew from experience what my speed was from the tach so it was no big deal to have a bad readout for the trip. The cruise still worked and is independent of the speedo anyway. I eventually got it pretty accurate through trial and error. I assume the interstate mile markers are fairly accurate and I was able to go fifty miles or so and still have the tenths read out within a few tenths of the one when I first started checking.

I still use the Pro Driver as the mileage because it has the maintenance reminder built in. It makes life simple.