NeilBlanchard wrote:The two ways to confirm it for a specific trip is to:
A) use a GPS unit to compare to the trip odometer
B) use Google Maps for the route to compare to the trip odometer
A third way is a little harder and slightly less accurate, is to drive on a highway that has marked distances. The hard part is you have to go far enough to "synch" to the error close enough to be able to be accurate. If it is over-reporting as you suspect, then you drive 30 miles indicated on the odometer, and it ticks over to 30.0 miles just as you pass the 29.3 mile marker.
I think the highway marked distance method would be the most accurate. GPS units calculate distance by taking the distance between points dropped every so often, and are not inherently accurate. Sort of like estimating a circle with a 100-sided polygon. Close, but not something to use as a golden standard. Highways, on the other hand, are carefully built and surveyed by engineers and surveyors with laser equipment.
In any case, 20-30 miles is FAR too small of a sample to be using. I would not estimate odometer accuracy with any distance shorter than 100 miles.