digital spedo making us crazy

our 1993 has the digital spedo init it never reads right up to approx 50 it read about 20 to 30 over what the car is moving. over about 50 actual speed the spedo read slower till you get to about 65 or 70 actual then it reads you are moveing 0 anyone have a clue what to do we have changed the spedo sensor on the trany this didnt help also went to parts store to have it checked with code reader this showed nothing

hey i work at a service center and theres a guy that comes in with his 97 dakota and 03 pt turbo but any way he uses this programer that agusts the comp along with the spedo i cant think of the name of the nifty tool i no its by hyper tech ill get back to ya with the name and exactly how it works