CYP77 - Police | Race Bikes | Choppers, Bobbers and Mods
Well actually both,
(Say) 2000 revolutions (per minute) of the crank becomes 300 revolutions per minute on the cable.
Then the reverse happens in the instrument, 300 revolutions per minute on the cable shows up as a reading of 2000 revolutions per minute.
The reduction ratio is just there to keep cable speeds within sensible limits.
Big British bikes (small ones rarely had rev counters fitted) were slower revving so a reduction of 2:1 or 4:1 was enough to keep cable speeds low.
Higher revving Japanese engines needed a higher reduction ratio 20:3 or 6.666:1 to keep cable speeds down.
If you can get a 7:1 instrument that will be close but the rev counter will actually read around 5% slow. Probably near enough, many rev-counters, aren't that accurate (within 10%) anyway. Hope this helps.