Speedometer Head: Adjustments

Speedometer Calibration
The speedometer is an instrument which processes information sent to it by the Vehicle Speed Sensor (VSS). If any of the system components send the wrong number of revolutions per kilometer (mile) to the speedometer, an inaccurate speed reading and amount of distance traveled will be displayed. Since there is no error in the fixed gear ratio of the speedometer gear, start by checking the accuracy of the odometer even if the customer indicates a speed accuracy problem. Odometer accuracy can be checked by using roads established at kilometer (mile) increments or a known local course. If roads with kilometer (mile) markers are used, a five kilometer (mile) stretch is recommended to allow for inaccuracies. If an error is greater than 3.75 percent, a change to the speedometer drive gear selection, tire size, or tire inflation may be required. The odometer should be checked again to verify any corrective action. If the indicated speed error exceeds 10 percent between 80-96 km/h (50-60 mph), replace the speedometer.