`95 Hardbody Speedometer Help

[misterZ]

My speedo was going out intermittently (a smart rap on the dash brings it back) so I bought a donor cluster out of a junked 95 Maxima. I replaced the (guts) speedo motor and circuit board on my Hardbody to get it ready for my son to DD; however, when I energized the motor to set it to zero and put the needle on, it's ~5-6 mph low (to the bad). The quick fix was to drive up and down the street and calibrate it against a police radar (digital) speed sign (moving the position of the needle trial & error). At rest, the needle is now on ~11 mph but under way is spot on 30 mph @ 30 mph. With ignition off it falls to zero (pinned).

 

Does anyone (experienced) have any suggestions or do you think the problem is the fact the motor and board are out of a car versus another truck? I can guess all day long but hope someone here actually knows why and how to resolve as this will be my son's first vehicle on a fresh FL license. =/

 

Thanks in advance!