Ok, that should be doable.
I think first you will need to get a handle on the rpm monitoring then go from there???
Here is a thread about that, it is old and if you have PBP 2.5 or newer then the LONGS option should help.
http://list.picbasic.com/forum/messa...tml?1030588166

in the FAQ section there is a post "better search ..." I think that will be handy also.

I have not done anything quite like this but I think I would do PWM based on RPM , maybe. I will have to think more about this....