Symptoms of a Bad or Failing Speedometer Sensor
Common signs include the speedometer not working, cruise control not engaging, Check Engine Light coming on, and loss of power.

Where is the speedometer sensor located?
The speedometer sensor is located in the transmission of your vehicle and is designed to register the driveshaft’s rotational speed. The sensor delivers this information through the speedometer cable and to the vehicle’s computer, which converts electrical pulses into a numerical speed. Eventually that number is displayed on the speedometer and tells you precisely how fast you are driving. If the speedometer sensor is not working properly, the speedometer will be inaccurate; potentially leading to speeding tickets or unsafe driving situations.

Speedometer does not work
Whether you have a digital or manual speedometer, if it's not displaying accurate information, it doesn't really help you. One of the biggest signs your speedometer sensor is not working properly is the speedometer will display inaccurate readings or the needle or digital numbers will not move at all on the speedometer display. A malfunctioning speedometer sensor can also cause a speedometer to act erratically, fluctuating the reading from one speed to another; even though in reality you're not speeding up or slowing down.

If you feel you are driving faster or slower than what the speedometer reads, it is a good idea to have your vehicle inspected by Your Mechanic, as this might be a problem with the sensor that can be repaired or an indication that the speedometer sensor needs replacement. Being proactive about repairing your speedometer issue can improve safety, not only for you, but others on the road. It can also reduce the potential of getting two citations; one for speeding and another for a damaged speedometer.