constant speed gopher?

I have a bicycle computer on my gopher to measure speed and distance. It is a reed type on 330x100 wheels. how hard would it be to make an auto compensator to buck/boost the set top speed to make it constant up and down hills? Wheel speed is abt 90rpm? I would want to lock it around 60rpm error <=+-2-3rpm. Do not know value of master speed control pot. Might need a linear ramping voltage which is applied to speed pot if timing wrong. adjustment by progressive floating increments. Will need PID control module. Could get $$$.