How do you fix the speedometer its reading 65 sitting still on a 90 gmc suburban?

To fix a speedometer that reads 65 mph while the vehicle is stationary, first, check the speedometer cable and its connections for any damage or disconnections. If the cable is intact, inspect the vehicle's speed sensor, which may be malfunctioning or faulty. You may also need to recalibrate the speedometer if it has been previously serviced or replaced. If these steps do not resolve the issue, consider consulting a professional mechanic for a more thorough diagnosis.