You haven't even begun to touch on this topic with this little sentence.

On Sep 3, 2007, at 6:43 PM, Wesley Pegden wrote:

> One thing to keep in mind is that many speedometers read a little
> fast.
> You should compare your trip odometer to highway mileage markers to
> see
> what factor it's off by to correct your numbers.

The US Standard is that speedometers read 3-5% below actual speed.
But odometers should be 1% or less from actual. But all that is moot
once you start going away from stock tires. And if you have gone with
larger wheels your figures can be all over the map.

In my case. I have one syncro, where the SPEEDOMETER reads pretty
much 10% low with the tires on it (when it says you are at 50 mph,
you are actually at 55mph.) The ODOMETER, on the other paw is 7% off.
When it says you have traveled 200 miles you have actually traveled
214 miles.

In another van, the numbers are different. And the tire sizes are
different. In that van the speedometer is 12% off, and the odometer
is 8% off.

Just because you have calculated the margin of error for your
ODOMETER doesn't mean you have calculated the margin of error for
your SPEEDOMETER.

My calculations of margin of error are based on GPS values for speed
and distance.