* Vehicle Speed Sensor (VSS) issue: This is the most common cause. The VSS is a sensor that measures the speed of the vehicle's transmission output shaft. If it's faulty, it might not generate a signal until a certain speed is reached, or the signal it generates below 40 mph is too weak or erratic for the speedometer to interpret correctly. The sensor itself could be failing, the wiring to it could be damaged or corroded, or there could be a problem with the connection.
* Speedometer cluster problem: The speedometer itself, part of the instrument cluster, could be malfunctioning. While less likely to be speed-dependent, an internal component might only function correctly after a certain voltage threshold is reached (obtained at higher speeds).
* Transmission issues (less likely): Problems within the transmission itself *could* theoretically affect the VSS signal, but this is less probable as a primary cause of a speedometer only working above a certain speed.
* ABS sensor (rare): While the ABS (Anti-lock Braking System) sensors don't directly power the speedometer, some vehicles use data from these sensors as a backup or secondary speed input. A fault in one or more of these could contribute to the problem, but it's less likely to be the sole cause.
In short: You need a mechanic to diagnose the problem. They'll likely start by checking the VSS and its wiring, and then move on to the speedometer cluster if the VSS is found to be okay. Ignoring the issue could lead to other problems down the line.
