a few thoughts :-
1, INTCON.7 "GIE" and INTCON.6 "PEIE" probably need to be set

2, this piece of code can never execute because its orphaned

RESUME ' Return to main program
ENABLE ' Enable interrupts


TMPlowbyte = 0 ' Variable reset
PIR1.0=0 ' Reset TMR1's Interupt Flag
TMR1Overflow = 0 ' Reset TMR1 Overflow counter


END ' End of program


3, the " on interrupt " latency is variable and unpredictable and is unlikey to give accurate results.
dt interrupts is vastly better.

4, for your purposes rpm measurement using "pulsin" is far simpler to do and a lot easier to get working.