RE: speedometer error

["John Dohrmann" <jdohrmann@domain.elided>]

To calculate your speed from the tach, you need your combined transmission
and differential ratio plus the actual circumference of your tire.  If you
are going to take numbers from someone else, check what tires they use.
For my '86 spider Quad, the combined gear ratios are 13.53, 8.159, 5.535,
4.1, and 3.239.  I went on the Pirelli site and got the turns per mile for
my tires.  After that, the math is easy.  I did a spreadsheet showing mph
for each gear for rpms of 1000, 2000, etc.  Interesting to see how much
bigger the changes are from 1st to 2nd and to 3rd vs 4th and 5th.
Speedo is very close to tach and dash tach is very close to shop tach.  I
got the GPS for the boat but it works well in the car.

John Dohrmann

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