Speedometer problems are as wide ranging as a simple loose wire to a massive electrical problem that requires the help of a professional. A jiggling speedometer needle or one that won't move at all isn't just an irritation; it's a road hazard. Whatever the problem may be, it is critical that a speedometer is fixed quickly and correctly so you know your driving speed. Though larger problems will need a mechanic, there are some problems that can be fixed at home with your own tools and a little elbow grease.
Lift the bonnet and look at the cord that connects from the dash to the transmission. Wiggle it to see if it has come loose or worn out.
Look at the speedometer head. Wipe away any collected grime or dust with a damp rag. Put a fresh drop of oil on the bearing.
Detach the cable from the casing on the transmission end if your speedometer is making strange "whirring" sounds. Pull the cable out carefully. Use a solvent to clean the cable. Let the cable dry, then rub the cable with non-migrating grease that is lithium based. Push the cable back through the way it came without forcing it. Turn it slightly when reconnecting it to the speedometer. Test drive your vehicle to make sure the speedometer works properly.
Ensure that you haven't recently changed to larger tires, changed your transmission or got involved in a rear-end crash, as these factors can all contribute to speedometer problems.
Check the gear at the transmission end of your speedometer cable to see if there's any damage caused. Purchase a new gear and change it if the part looks damaged.
Compare the speed of your car to the speed of other cars on the road while driving to see how far off your speedometer is. Take the car to a speedometer shop or someone who knows how to fix speedometer magnets to recharge the magnet.
If these do-it-yourself repairs don't work, the problem is most likely electrical and will have to be repaired by a professional. If you don't want to check your speedometer while driving, you can purchase an Odometer Calibration Form to check the car while standing.
Remove all cables very slowly so the wiring is not damaged.