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The New Vintage USA Woodward series gauges that were installed in my '52 before I bought it use a replacement unit for the speedometer/odometer cable. It sends electric pulses to the speedo head that times them and sets the speedo needle and odometer.

The "calibration" routine is:

1. Stop on the Interstate shoulder at a mile marker.
2. Put the unit into "calibration" mode.
3. Drive exactly 1 mile and stop at the next mile marker. Speed doesn't matter, the speedo head is just counting pulses from the sending unit on the transmission.
4. Tell the calibration mode that you have driven one mile.

The speedo is now ready for use. It has stored the number of pulses and will use that to move the speedo needle and the odometer numbers.

In my case, the stored number was just a hair over 10,000. That is just about 2 ticks per foot.

The speedo gives you an option to change this number up or down, one pulse at a time if you need to correct anything. I went to another mile marker and tried the calibration again. This time the count was about 100 less than before. That is a 1% difference. Not a big deal unless you have a slight case of OCD, like your's truly. I "adjusted" the calibration up 50 ticks and called it a day.