Speedo gear chart vs. math...

think about it; the more teeth on the pinion, the longer it will take to make one revolution, and so, the slower the speedO will read.

The speed-o needle is driven by magnetism; it has no connection to the cable whatsoever; the cable just drives a little generator. The generator creates the magnetic force to magnetically drag the needle ever higher with increasing rpm. Anything that affects the generator rpm, affects the needle reading............ unless the needles speed-cup is damaged or dragging or it's bearings are loose.

If you knew the correlation between cable rpm and needle reading, you could drive the cable up to speed with an electric drill, and prove the units accuracy. I don't know it; maybe someone here on FABO knows.