Today, I stuck the GPS onto the windshield and took the truck out on the highway. At 90 KPH (about 55 MPH), the speedometer indicated 40 MPH; this speedometer error is pretty close to what I calculated it would be. Engine RPM is about 1750 at this speed in 4th gear.
Rear axle ratio is 2.41. Tires are 767 revs/mile.
