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Seeing a different speedometer speed vs the GPS or OBD speed?

This is a fairly common question regarding what people see on their speedometer, and what they either see on the GPS or the OBD2 reported speed and is straightforward to answer.

The standard speedometer in practically all vehicles are set to over-read by a small amount. This is done at the factory and is perfectly normal.

You can see how much your speedometer is calibrated to over-read by, by having a passenger add the adding the 'OBD speed' widget to the torque display and comparing it against your vehicles instrument cluster speed whilst the vehicle is being safely driven at a constant speed.

You can also see how much out your ECU speed measurements are by adding the GPS and OBD speed widgets next to each other, or logging them whilst safely driving at a constant speed. Typically from my own testing they are usually very close (typically less than 0.5mph off).

A typical cause if the readings are more than 0.5mph out would be different diameter wheels than the factory specification being fitted.

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