speedometer registers faster than the truck is moving
check the automobiles tyre size is correct as per the original specification, also I would get a qualified technician at your local shop to calibrate the speedo as it wouldn't have been calibrated from new.
This may be normal. Back in the early to mid '90s GM was calibrating speedometers to read slightly higher than your actual speed. This was to help avoid speeding tickets. My '93 Caprice read 4 MPH higher than I was really going. I bought that car new, BTW. HTH. -Jim
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