After much messing around, testing, and long conversations with the manufacturer of this Hubodometer, I am giving this up as a failure. The problem with this unit is, in spite of being programmable for small tire sizes, it is really designed for large semi sized truck tires. It is accurate at low speeds, but at freeway speeds it just stops counting because at freeway speeds the RPM of a small size tire exceeds the frequency response of the electronics.
Back to the old fashioned recording of the TV odometer. Oh well.