I guess speed is important, but isn't cadence even more important? The whole thing about going fast is building revs. Knowing what you are spinning can also help you fine tune gearing.
Cadence is also a much more accurte measure of speed. On my road bike, the speed will be, for example, 23.7 mph and will stay right there even though my cadence varies 5 or so rpm.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1