Test your speedometer for accuracy with a stopwatch while driving a
steady-state speed between mile markers.  Mine reads 5 mph slow, making it
a real ticket-getter.

With electronic speedometers now in current  use, the manufacturer could
incorporate a simple, user calibration feature.  This feature would be
accessed by the driver.

Given todays digital electronics, the error induced in the speedometer
comes not from the instrument, but from the varying rolling radius of the
driving wheel.  In fact, additional error is introduced by the differential
gear as it accommodates driving wheels which may differ slightly in rolling
radius.  A simple calibration feature could reduce speedometer error to no
more than 0.5% with negligible cost to the manufacturer.

Here's how it can be done:

The only parameter that need be measured by a speedometer calibration
microprocessor is the number of wheel revolutions per mile.  Currently,
this is an estimate based upon the installed tire-wheel combo.  Revs/mile
can be counted by the transmission with a little input from the driver.
All he needs to do is to press a button at the start of a mile marker and
again as he crosses the next mile marker.  For more accuracy, one, two, up
to five mile markers can be used and the computer will recognize the larger
intervals and calculate accordingly.  The speed of the car, or speed
variations, during measurement is irrelevant.  The computer just needs to
calibrate the wheel radius, as the rest of the speedometer calculation is a
simple algebraic calculation based on time, which the microprocesser is
accurate to microseconds per minute.  With the button presses, the computer
is signaled with the elapsed distance information as it counts the
revolutions.  It then knows revs/mile.  In real time, it knows time via its
internal clock and calculates distance / time = velocity on the fly to read
out miles per hour.

So for example:  You buy a new set of tires.  Take the car out on the
highway and at the first mile marker, press the cal. button.  At the second
mile marker, press again.  The speedometer is now calibrated.  

Note: In the case of gross operator error say exceeding 15% of the preset,
the speedometer would not reset to some absurd miscalibration, but would
simply go to a preset default and prompt the driver to redo.  A variation
in control would be a pair of buttons, one button to start the calibration
and one to set it.

There is no downside to my suggestion and I will expect to see it in next
year's higher end models.  There is no longer any reason for miscalibrated
speedos as they can lead to tailgating (highreading speedos) or tickets,
fines, increased insurance rates, and in the event of fatal accidents,
criminal charges of vehicular manslaughter (low reading speedos).  Anyone
who suffers losses from inaccurate speedo indications after next year's
models should consider adverse action against the manufacturer, now that he
has been given fair notice of the solution to the high or low reading
speedometer problem.