Skip the auto shop - Our mechanics make house calls

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.

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Tires are vibrating

The tires would need to checked to see if any damage to the tires is found and if the tires are properly balanced. If you added any tire sealer to a tire to try and seal a leak, then the...

Q: Brake Pad Wear Alarm& warning light appear in dash, replaced all pads and wire sensor but unfortunately no any positive result 2012 Lexus LS460

Hello - unfortunately, this warning message can only be cleared by the Toyota/Lexus diagnostic tool, or the "super secret" push-button sequence which the Toyota techs aren't sharing.

Q: The oil pressure gauge triggers the red oil lamp when idling

This may be related to low oil in the motor or potentially a faulty oil pressure sensor. I would recommend checking the oil via the dipstick first to be sure the proper amount of oil is in the motor to...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...