Jack the car up, rotate the wheel 1 revolution, count the number of turns out from the speedo drive.
Drop the car back down, mark a line on the ground and a chalk mark on the tyre, roll the car so the wheel goes 1 revolution.
Measure the distance travelled for 1 rev.
Using a bit maths jiggery pokery, you need to divide 5280 feet ( 1mile) by the distance travelled to find out how many turns the wheel makes per mile vs the speedo drive revs.
Did have the exact formula written down someplace but its plenty do-able.
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