speedomoters use either a hub-based, transmission-based or differential-based speed sensor. not from the computer. the information may pas through the ECU on way to the instruments, but it can't tell the speed itself.
if you want to know if the speedo is accurate you'll need to test it with a GPS-based system or a rolling road.
if it's mileage you're talking about, then most new digital odometer can't be reset, but old analogue ones can be wound back easily, and there's no way to tell the original or true mileage.