Joey,
The problem is that you need to match the turns-per-mile that your speedo is
designed for, with the number of turns per mile that the speedo cable does.
The speedo cable's turns-per-mile figure is determined by the tire
circumference, the diff ratio, and the speedo gear ratio. If you change the
tire size, or the diff ratio, then you need to change the speedo gear ratio
to compensate. It's much more expensive to change tire size or diff ratio
because you've got the wrong speedo gear ;)
A second issue is the speedo itself, which is really two instruments: the
speedometer and the odometer. The odometer should advance exactly one mile
for every set number of speedo cable turns; that's determined by a fixed
gear ratio inside the instrument, and it doesn't usually break down. The
speedo function depends on having the right spring tension and magnetization
of the drive cup, and that's something that can easily be damaged.
Theo
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