You could download the Lite version of proton+ and use the following. Sure you loose the functionality of being able to do things for a certain period of time with this command, but you only need to sample the signal then scale it up.
Syntax
Variable = COUNTER Pin , Period
Overview
Count the number of pulses that appear on pin during period, and store the result in variable.
Operators
Variable is a user-defined variable.
Pin is a Port.Pin constant declaration i.e. PORTA.0.
Period may be a constant, variable, or expression. (In milliseconds)
an example would be;
Dim Variable as Word
Main:
Variable = Counter, PORTA.0, 100
Print At 1, 1, "RPM = ", Counter * 100
Goto Main
To improve accuracy, simply increase the sample time.