There is often a difference between the odometer reading on the dash board and the odometer reading on the truck ECM or Electonic Onboard Computer. This has to do with the fact that the ECM reading is pulled from the GPS pings and the dashboard reading comes from calculating how many rotation a truck's tires do within 1 mile.
How much of a difference and how much is too much?
The difference typically falls in the range of 1% - 18%, and should be corrected if the variation amount to over a 20% difference. This is between 80 and 100 miles.
Which reading is accurate?
The ECM is the most accurate. It typically runs off of the trucks GPS system and as a result provides a more accurate reading for the number of miles traveled by the truck.
How does the variation happen?
The variation can be connected to a number of different factors.
- A wheel spin out in mud or on ice;
- Tire inflation is not maintained properly;
- A Tire is damaged on the outer rubber;
What does this affect?
It only affects carriers that run their reported miles directly off the odometer readings the drivers are reporting in their log books which these days are far and few between. It will also likely be looked at during a some kind of mileage audit (example: IFTA) where the mileage reported by the drivers on the logs (reported from dashboard) will be compared to the miles reported from the ECM and the GPS pings sent out by the truck. Typically auditors will take into account the usual variance.
What can be done?
The only thing that can really be done is to try and catch the problem before passes the 20% mark and get it corrected in the problem vehicle.
The previous information was somewhat incorrect.
I was under the impression from someone that I considered a reliable contact that the difference could be adjusted. This was in fact wrong. Freightliner has informed me that adjusting the odometer is illegal and I am still researching to find what should be done when this variation happens. In the mean time I recommend just taking note of the adjustment in case of an IFTA audit.
I have found out how the problem is fixed.
If there is a variation between the odometer and the ECM, the ECM is replaced so that the numbers match.
It would be more accurate just to make an adjustment on the odometer reading by altering the odometer to match the ECM which is the more accurate of the two. However, because this is illegal, the ECM must be switched out to ensure the two match.