I've replaced the Mini's mechanical speedometer with an electronic unit from a Mitsubishi FTO. This speedometer requires 2548 pulses per km, but my motor controller outputs a 7031 pulses per km. I need to make a box that outputs 1 pulse for every 2.764 input pulses. I made a fairly naive 2.75:1 implementation in an arduino by outputting one pulse for every 3 input pulses except every 4th pulse, where it only waits for 2 input pulses. This jitter doesn't seem to affect the speedo needle even at fairly low speeds.
The current implementation counts to 11 and uses a switch statement to output pulses which isn't really generalizable to other ratios but is probably close enough to what I need.