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Q: Speedometer doesn't register correctly

asked by on

When I go say 60 on the speedometer in my cherry 2004 Police Interceptor, my GPS says I'm going 70. (30 is 35, 40 is 46, 50 is 57 and so on). I discovered that someone, before I bought it at auction, had to have changed the complete rear end assembly. The Oasis report said my PI came off the assy. line with 3.27 gears but a tag on the rear end cover says 2.73. Also there are tell-all markings on the axle housings in "junk yard" white chalk. Makes sense it was changed for some reason. Can I just change a gear in my VSS sensor or must I need a completely different sensor to match my rear end ratio to my speedometer reading? Thanks so much.

My car has 60000 miles.
My car has an automatic transmission.

A: Hello and thank you for your question. When...

Hello and thank you for your question. When it comes to a speedometer not working correctly, then there can be two things that cause this issue. One is if the VSS (Vehicle Speed Sensor) in the transmission output shaft has failed and is not registering correctly. Remove the VSS and check the gears on it. Place the sensor in a vise and spin it up with a drill. At the same time, check to see if the sensor is producing a voltage. If the sensor does not produce any voltage, then the sensor has failed. If the sensor produces voltage, but is less than 5 volts, then the sensor has failed.

Also, the tire size on the vehicle will change the speed of the vehicle. For example, larger tires will make the vehicle cover more ground at a given speed, making the vehicle go faster than the speedometer, and smaller tires will cover less ground, making the vehicle slower than the speedometer. Look at the door placard and make sure that the tires are the same size as the recommended size.

When changing the differential gears (final drive), this changes everything. When the gears get bigger, there is more torque to the wheels, but this makes the wheels turn slower. Even though the VSS is reading the transmission output shaft speed, the differential is turning at a slower speed than normal. You cannot adjust the VSS; however, you can get a VSS that will match the gear ratio for the speed you need. I recommend seeking out a professional mechanic, such as one from YourMechanic, who can determine the exact reason why the speedometer is not working correctly.

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