Garmin to the rescue at least for me.  I use a GPS to measure speed. I
visually compare with the speedo reading and then generally decide to leave
the gps on.  Accuracy is around +/- O.3mph.  Use it once at 30, 50, 60, 70
(GPS speed).  Note the readings on your speedo.  Next do 1,10,20 miles and
note the readings on the odo.  If the odo is spot on the speedo itself needs
adjustment.  If they're both off by the "same" amount a gear change (in the
speedo/odo is needed to calibrate the unit to the rotations / mile of your
tires.  NB use the same tire pressures and loading for each test.

And in a pinch, when in your quest for the truth you forget to fill the
tank, you can use the position to tell the Car Clubbie where you are.  If
you have a GPS V you can plot him a route to find you too.  AND you can use
the critter as an emergency chock, throw as a rock, keep the time like a
clock, measure how far it is around the block, calculate a route to the
nearest auk and so on.

pensioner