Andy,
This was an issue because of a speeding ticket I got,
had to go get a certificate of calibration from a
Licenced Speedo shop, take it before the judge.
FWIW, using the formula, gets 18 MPH/1000 RPM with
4:11 rear and 25" dia tires. So it depends on the size
of your thumb.
So I used the 'conservative' number (20) thereafter,
because it meant that at indicated 2,000 RPM, I was
actually a bit below the 40 MPH speed limit. Beats
doing floating point math in my head while driving.
Can also just put little white tape markers on the
tach.
This makes a difference if you live in 'revenue
hungry' or 'zero tolerance' jurisdictions.
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