speedometer calibration

[projekti]

I am lucky as the company I work for had a gauge repair shop in the 70- and 80īs, with all the tools and calibration machinery still in working condition! ( just nobody around to tell how they work)
When I hook up my 1973 T speedo to the Smits calibration machine it tells me that at 1000 rpm the dial shows approx. 70 mph.
Does anyone know how to calculate the rpm at the speedo into MPH ?
The gearbox is a 915 , 5 gears. Tires 195/65-15