Programming – Vehicle Speed Calibration

The value (Pulses per Mile, PPM) of this parameter is used by the ECM to convert the vehicle speed signal into miles per hour (or kilometers per hour). The value is calculated using tire revolutions per mile, rear axle ratio and the number of teeth on the transmission chopper wheel. This parameter is programmed by the OEM.

PPM = M x Ra x N
Example: 31,200 = 502 x 3.90 x 16

M = Tire revolutions per mile. This information is available from the tire manufacturer.
Ra = Rear Axle Ratio. This is typically found on the housing of the rear axle, or on the specification sheet for the vehicle
N = Number of chopper teeth on the transmission drive shaft where the magnetic pickup is mounted.
The value is typically 16, but may be 11.

All electronically controlled on-highway engines


Proper calculation of the Vehicle Speed Calibration parameter is essential for proper cruise control and speedometer (if controlled by the ECM) operation, and accurate Fleet and Driver Trip Data.

Miscalculation of the Vehicle Speed Calibration parameter will cause all values calculated by the ECM using the Vehicle Speed value to be in error (e.g. vehicle miles, fuel economy, etc.). Precise calculation is critical to provide accurate calculated values from the ECM.

Let the OEM calculate and program Vehicle Speed Calibration parameter based on the truck specs.

Recalculate and program the Vehicle Speed Calibration parameter if any changes are made to the driveline components, such as a different rear axle ratio or a tire size change.

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