I made my own measured mile by driving another car with a good speedo along
a local road, and noting the location of 2 landmarks. Then I drove the car
in need of calibration alond the measured mile and noted any differences.

So now you have two sources of error. How do REALLY know your benchmark
car has an accurate speedo? It could be reading a couple percent off in
one direction and the other car a couple percent in the other direction
and you really don't have a problem at all. You need to use a known
standard. ie Mile marker signs on the expressway or borrow a GPS.