My speed reading is off and slowly increases as I go faster per my GPS speed. When I am going like 65 my speed shows as about 85, but when I am going like 30 it shows at about 38. I have a brand new cable, so I suspect the speedometer is what is bad. I have an extra one, but the miles is 54k and my truck only has 31k. So when I am on the highway I just use my GPS for my speed.
It should be directly proportionate to your speed, say it reads 66 on the speedometer and it is actually running 60, at half that it should be 33 and 30. Ten percent in this example. There are some exceptions though, if the cable is binding and snapping forward, the needle will read much higher than speed as it is kicked up by the faster pulses.
All three of mine read faster than they are actually running. It may be where some of the M1009 guys are saying they run 75 and the engine sounds fine, because mine are both way off. Good for the 6.2, let them think they are running 75-80 while it is quietly humming away at a much lower rpm.
Oddly though, the odometers are very close. On my M1008 it takes about 35 miles to gain a 1/10th of a mile on the Interstate. When I came home from Utah, I knew my speedometer was way off, but I could not find error in my odometer.
Glen