Eric:
A non-electric odometer "gets its numbers" from the same cable that drives the speedo. The number of teeth in the gears that drive the cable, move the odometer etc. are related to the OE tire size and thus you get a proper reading. When you change to significantly larger tires, as on a Jeep, you then must replace the gear on the tranny side of the cable to account for the difference in travel per rotation, or your reading of both speed and distance will be off (low). Now if your talking about electronic speedo's (don't know what kind of car you are referencing but your language leads me to believe you are), I do not know for sure but I would assume it is a speed/time calculation made by a computer, as most of those cars can use the same calculations to give you range, average mpg, average speed, etc.. HTH

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick