Could OVMS create a 12v squarewave pulsed output at approximately 1 pulse per 1 MPH? canm8 would like to use OVMS instead of their CAN Bus Speed Pulse Interface http://www.canm8.com/can-bus-interfaces/speed-pulse-interfaces/canm8-pulse.html
The CANM8 speed interface is used in many Taxi cars that use the pulses for the taxameter.
It seems overkill, but technically possible with an expansion board.
The ESP32 can certainly do PWM output on its GPIO pins. setting the PWM based on speed metric is trivial.
The only issue I see is converting the logic level GPIO PWM signal to 12v. That would need a small chip on an expansion board (or external connector). We already do that for the switchable 12v output we have, so not difficult.
P.S. The existing switchable 12v output could probably not be used for this. That is driven from the MAX7317 GPIO extender, which doesn't have PWM capability. Sure, you could drive it in software, but at 100mph that would be 200 cycles a second - possible with custom dedicated firmware, but unlikely to be accurate when running all the other stuff like wifi stack, can buses, etc.