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A rolling radius or gearing that does not match what the module expects, so calculated road speed no longer matches actual road speed. It is distinct from mismatched tyres across an axle — here every wheel agrees with the others, and all of them disagree with the vehicle's configuration.

The module derives speed from wheel or output shaft rotation and a stored constant. Change the tyre diameter or the final drive and that constant is wrong, so indicated speed, distance and every speed-dependent function drift by a fixed percentage. The signature is a proportional error rather than an erratic one — a speedometer consistently reading a few percent out is this, not a fault.

Modifications and part substitutions are the routes in: a plus-size wheel and tyre package, a differential swapped from another model, or a transfer case with different gearing. Compare indicated speed against a satellite reference and calculate the ratio, which identifies it in one test. Many vehicles allow the constant to be configured, and where they do, correcting it is the repair rather than reverting the modification.