VSS and Distance Measurement
Good Evening Alexandre,
I apologize, I couldn't be more helpful.
Thank you again.
OK, thanks. US Federal requirements only require that the VSS reported via OBDII are to nearest 1MPH (or 1KM/Hr) and that the parameter is updated every 100 milliseconds. Of course the original VSS analog signal is a square wave pulse train which, in essence, is super accurate since it is measuring revolutions of a shaft (generally) in the gearbox. Thus there is virtually no error there assuming you know the amount which the vehicle moves per pulse, and that is a fixed amount if we ignore expansion of the tires, etc. Of course when the OBDII signal is reported it is an approximation of the VSS signal frozen and reported 10 times per second, and then only to the nearest 1MPH (or 1KM/HR). So it has a variance when compared to the "count" of the original VSS pulses. It is a shame that the standard does not require reporting of the total pulses which occured on VSS SINCE THE LAST REPORTED VALUE. In that way a more accurate approximation of speed could be calculated as well as a very accurate estimate of distance travelled.
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