I just bought this car so I dont know the history Speedometer reads maybe 10 MPH too High, looking at the board I see fixes for this problem by changing the gears for the sensor in the tailshaft and I see a fix by programming, which is the way to go. I feel maybe the tranny has been changed in the past but I dont know for sure. I also dont think this problem is linear that is the faster you go the more wrong the speedometer is off.

 

What size tires are you running?

 

P235 R55 16

 

Have your local tyranny shop do a free inspection and tell you your rear gear ratio. Compare that yo your original ratio. Also have the see if the tyranny is original or not. I would myself prefer to change the gear in the transmission in conjunction with tires to correct the problem.