How about using a frequency divider circuit like a counter. For arguments sake a decade counter whose ouput is DCBA(D=MSB A =LSB). The pulse from the motor(assumed to be accurate) is given to the input of the decade counter. The MSB can be used as the gate pulse to the 8051 counter. The RPM is calucluated on a timer interrupt. The 8051 counter reading and the reading of the decade may be used to "accurately" determine the RPM (Haven't figured the error yet). This is just an idea. Probably a stale one!!