John,
The 'formula' you cited/quoted is correct, but only for tires sized at 810 revs/mile or so ... 225/55ZR-16 or 245/50ZR-16 ... to use your own tire size, you would need to look up the specs at Tire Rack and substitute in the formula below for the 844 revs/mile I used.
For example, Yokohama A008P 245/45ZR-16 tire have 844 revs/mile per the Tire Rack spec page ...
The sensor for the electronic speedos is a simple pair of 'dry' [no Voltage present] magnetic reed contacts ... which can be simulated by a reed relay driven at 120 pulses per second by a wall transformer and full-wave bridge rectifier, which equals 7200 pulses per minute.
Using the A008P example ... 844 revs/mile x 8 = 6752 pulses/minute for 60 mph calibration. To use the 120 pps source, for the A008P's ... you would adjust the speedometer to: 60 mph x 7200/6752 =
64 mph.
The following diagram shows the speedo circuit and has the sensor connected to terminal 31b & Gnd.