NAVGPS

How-To

Set actual miles so OBD2 speed matches reality

Your vehicle calculates speed from wheel rotations and the factory tire size. Change tire diameter and every OBD2 speed and distance figure is wrong by the same ratio. NavGPS fixes that with one drive between two mile markers.

Before you start

  • An OBDLink MX+ connected, with the engine running
  • A stretch of road with mile markers, or any two points a known distance apart

Steps

  1. Open Speed Calibration
    Engine tab → the ••• menu → Vehicles → Speed Calibration.
  2. Start at the first marker
    As you pass the first mile marker, tap Start at the First Marker. NavGPS starts adding up the distance your vehicle reports.
  3. Drive the known distance
    Hold a steady speed. The distance counts up on screen as you go — this is what your vehicle thinks it is covering.
  4. Stop at the end marker
    Tap Stop at the End Marker as you pass it.
  5. Enter the actual miles
    Type the real distance between the markers into Actual distance. NavGPS divides actual by measured and shows the correction factor.
  6. Save the calibration
    Tap Save Calibration. OBD2 speed is multiplied by that factor for this vehicle from now on. The factor is stored per vehicle, so a second truck keeps its own.
The NavGPS engine dashboard with live readings from a connected vehicle
Speed Calibration lives behind the Engine tab's menu, under Vehicles.

Worth knowing

  • A factor above 1.00 means your tires are larger than stock and the vehicle was under-reporting. Below 1.00 means smaller tires.
  • The current factor stays visible on the same screen, with Clear Calibration if you go back to stock tires.
  • Longer is better. Ten miles between markers gives a far more accurate ratio than one.