Instruments - Maximum Permissible Speedometer Deviations
SI B62 06 12
Instruments
April 2012 Technical Service
This Service Information bulletin replaces SI B62 02 96. SUBJECT
Maximum Permissible Speedometer Deviations
MODEL
All
SITUATION
The vehicle speed displayed in the instrument cluster displays a speed that is higher than the actual speed.
INFORMATION
Most speedometers have tolerances of approximately +10%. A "speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, tire temperature, tire pressure, vehicle load and other factors.
Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle.
Excessive speedometer deviations can come from several causes, but most commonly are due to nonstandard tire diameter and wheel size. This SI defines the maximum and permissible speedometer reading tolerance.
The instrument cluster is a sealed unit and cannot be calibrated. Any tampering to the cluster will void the related component or vehicle warranty coverage that is applicable.
GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of deviation as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time.
If you are using a known accurate GPS system as a reference speed, you need to use the formula below to calculate the maximum permitted speedometer advance, which is 10% of the actual (input) speed plus 2.4 mph.
Example:
Actual speed = 50 mph
50 mph x 10% = 5 mph; 5 mph + 2.4 mph = 7.4 mph Therefore, the permissible displayed speed is 50 to 57.4 mph.
(Note: The displayed speed must never be less than the actual speed.)
No repairs should be attempted if the speedometer advance is within 10% + 2.4 mph.
Use ISTA ((Integrated Service Technical Application) diagnosis or the instrument cluster self-test to make sure the needle for the speedometer is moving smoothly.
NOTE:
The amount of speedometer advance has no effect on recording accumulated mileage in the odometer display. The odometer records total mileage digitally, and does not incorporate any "advance" tolerances.
