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The speedometer in a car runs based off the speed of the transmission. The gears in the transmission spin at a known rate for the speed the vehicle is traveling at. The faster the gears spin the more the speedometer goes up. If a person replaces stock tires with larger tires, then the tires will spin slower, which will cause the speedometer to read at a value that is less than the actual speed.
Usually, stock vehicle speedos will show a little more than actual speed. But if the vehicle has been modified, all bets are off.
Is the TCM working normally and throwing no codes?
there is a speed sensor on the rear of the tranny which puts out a pulse to the ecm this is converted to dc current to drive the speedometer faster the speed more pulses more pulses the higher the dc the higher the dc the faster the speed display
It does not change the actual speed of a vehicle but changing tire size can affect the accuracy of the speedometer.
Have the speed sensor replaced.
It is either the speed sensor or the wiring. Have it checked out.
If the speedometer and odometer are both not working, I would blame the speed sensor. If only the speedometer does not work and the odometer does work, the cluster is bad.
Yes, it is obvious that if the radius of tire is small than it effect on the speed of the car. Smaller tires will show the speed on the speedometer more than actual.
Larger tires will make the speedo lower than actual speed.
A hair over 78 mph (78.125).