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In Reply to: They use different sensors posted by Michael W on September 11, 2000 at 00:05:35:
The signal comes from the same place, however, the inaccuracy is, to a point, programmed into the needle, but not the odometer.
Tyre size will ALWAYS effect speedometer accuracy, and odometer accurancy.
I asked this question was and was explained that the data for the MPH speed and the trip computer/odometer data come from entirely different sources. I don't remember the technical aspects behind it, but the sensor for the odometer knows how far you have gone to a blindingly accurate degree but not "time it" so does not know your speed. It is not affected by tire size, etc, whereas the speedometer is.
One way you can test the effiency of your odometer is by playing with a stopwatch on an empty interstate and watching the mile marker signs. Set it up so that you are passing the signs only at 60-second intervals (regardless of the data indicated on the speedometer). Then set the cruise, once you verify that you indeed are accurately passing the mile markers at the precise spot, check the accuracy of the trip computer.