Actually Dennis, repositioning the pointer provides a linear solution, for
those cases where a linear error is noticed.  In many cases the error is
proportional to the speed or a non-linear percentage error.  EG:  A 5% error
will give a 3mph high reading at 30mph and a 6mph high reading at 60mph.
Repositioning the pointer will yield only a linear correction.  On some
speedometers the spring is slightly adjustable.

The speedometer uses the magnetic coupling between a rotating magnetic disk
and an iron cup to produce a torque that is proportional to the rotational
speed (angular velocity) of the speedometer cable.  This torque is resisted
by a spring attached to the needle assembly/cup in such a balance that the
angular deflection of the needle when mapped to the graphic on the face of
the of the reads the miles per hour.  The graphic could easily be calibrated
to read wheel rotations per second or furlongs per fortnight without
changing the mechanism.

Try a good shop, expert in these things.  WWW.PASPEEDO.COM is a good one.