The easiest and most accurate answer for you is to contact the tech support group of the Tach/Speedo manufacturer and ask to get/buy a copy of their maintenance specs used for calibration and repair of the instruments. Most electronic instrument service benches use fixtures on the bench for calibration, and the parameters should be available.
As an aside, I chuckled reading about your project; it reminded me of several projects I worked on as a young EE... I was designing digital instruments and built a digital speedometer for my '70 Barracuda. I used a strobe wheel from a tape transport, attached to the speedometer cable, as an interrupter for a photocoupler. Lacking any data, I had to experiment to find the conversion factors (# of pulses per unit of distance). (...aah, fond memories of SN74xx discrete chips).
I painted distance strips on the street in front of my house...marked the tires, made a wheel-shaped readout for the transducer and kept rolling the car up and down the street, taking data points.
Had all the neighbors peeking thru their curtains, trying to figure out what was going on.
I think the "great" revelation was that the cable spun 1000 times per mile, or something like that.
