Vehicle Speed & Distance Calibration

Elements influencing the accuracy

1. Load vs. No Load

When a truck is laden, the bottom of the wheel is deformed.

This deformation changes the effective wheel diameter.

If the wheel is compressed by 25mm, the effective diameter change by 50mm.

Let us assume the wheel diameter is 1000 mm with no load.

The wheel circumference will change as follows:

• There will always be a difference between the distance and speed of a vehicle with a load and without a load 
• Referring to the example above, a laden vehicle will record 10 520 km over a distance of 10 000 km if the vehicle was calibrated with no load.
• An unladen vehicle will record 9 480 km over 10 000 km if it was calibrated while being laden.
• A laden vehicle will record 10 000 km over 10 000 km if it was calibrated while being laden.

Best practice would be that the vehicle must be laden and

with the correct tyre pressure when being calibrated.

This will give the operator the most accurate readings when goods are being transported.

2. Tyre pressure

The same rules as above apply to under inflated and over inflated tyres.

3. Tyre temperature

As the tyre rotate during operation, heat is generated on the inside of the tyre. As the air temperature inside the tyre increases, the inflation pressure also increases.

Thus a tyre inflated to 80 psi cold would now be at 85 psi and this will influence the profile of the tyre.

Other heat generators are hot tarred surfaces and brakes.

4. Tyre size

The nominal tyre size can differ; especially when one use different manufacturers’ tyres

If the tyre size is changed, the calibration will also change as in the example above.

5. Calibration Distance

The margin of error decrease if the distance increase when calibrating a vehicle.

All vehicle makes do not have the same calibration figures, they can vary from 2 000 to 23 000 pulses per km.