Incorrect Calibration
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 1991 GMC Pickup, 1991 GMC Cab & Chassis, 1991 GMC C3500 HD, 1991 Chevrolet Pickup, and 1991 Chevrolet Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
Check rear axle ratio and rear tire size listed on factory identification label on glove box door. If rear axle ratio and tire size are same as factory equipped, check vehicle speed sensor circuit. See VEHICLE SPEED SENSOR TESTING & DIAGNOSIS . If vehicle speed sensor circuit is okay, check speedometer calibration using correct tire size. See SPEEDOMETER CALIBRATION under ADJUSTMENT.