Oh, in that case there's no need for the 18F2431, it would be overkill.
Set one of the PICs timers up as a counter and feed the pulses from one channel to it, then read its value at precise intervals and you have the velocity. Or use the Count command to Count pulses for a certian amount of time. Or, instead of counting pulses, use the capture module to measure the length of ONE pulse. Or use PulsIn to do the same thing, or.....

/Henrik.