Since my recent upgrades, a work still in progress at great personal expense
in time and money, I use my GPS to measure speed and distance travelled.
Wonderful device Garmin GPS III+.  Accurate enough to give bin
whateverhisnameis an M40 enema.   My speedo is off ~5% but on the low side.
3000rpm in top reads right on the GPS and low on the speedo.  The odo also
reads a bit low when compared against the trip computer mileage on the GPS.

Now I can't imagine that the cosine error or curvature error to be
significant in 350 or so miles but hey maybe it's not only not flat but
shaped like a Klein bottle or an inverted toroid or dependent on mood and
attitude.

Perhaps in rural free Idaho, where the fishing is fine and the sheep are
nervous, gravity warps, exceptions to the unified theory, or something else
like cold fusion will make significant deviations from normal a casually
observable space time phenomenon.

With apologies to Drs. Pollard, Einstein, and Feynmann.

pensioner (not talking about wheels today)