In order to determine what is the right speedo gear, you need to know the
effective rear tire circumference and the differential gear ratio. In any
case, switching from the 15 to the 16 tooth gear will change the speedo
reading down by about 6 percent (i.e. if it read 100 mph at a given road
speed before, now it would only read about 94 mph).
My Tiger currently had 205/60-13's and 3.31 gears, and with that, the speedo
reading should be just about exactly double the tach, as you describe, in
fourth gear. What I actually get is more a function of the speedo's internal
condition...
There is an Excel calculator for the speedo gears on the Tigers United
website, IIRC.
Theo
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