so the user's guide to every speedometer has a little chart giving the circumference for common tire sizes.
But the exact circumference (almost always listed in millimeters) for a given tire size is not consistent across the various speedometer user's guides that I have.
What I'm wondering is, is the variation actually meaningful? For example, say I have two bikes with identical 700x25c tires. One speedometer says to program 2124mm circumference, another says 2146mm. Is this meaningful? that is, does the speedometer have something else going on in its programming that I should plug in the number specified?
Or are all speedometers just multiplying wheel circumference by the number of rotations to get distance?
I assume the latter, and the differences in listed circumference are just because of variations in the tires that the manufacturers measured, or because they were just sloppy in producing the chart. But I'm curious to hear if others share my assumption.
Just for the record, here's an example of the existing variation. Circumference measurements for 700x25c tires from the various speedometer user's manuals I have...
2100 (Cateye Mity 2, from about 1995)
2105 (Filzer wireless)
2110 (Cateye Mity 8 and Cateye Velo 2)
2120 (Planet Bike Protege)
2124 (Ascent wireless)
2146 (E3 and Axiom, which I think are both Performance in-house sub-brands)