Here's an explanation of the RPM setup

13 NEWRFPER PRESENT MINOR LOOP REF PERIOD MSB
14 NEWRFPER+1 PRESENT MINOR LOOP REF PERIOD LSB
mS = N/65.536
RPM = 1310720/N

N is a raw value from the ECM which represents the number of ms between reference pulses.

Examples:

When N=1:
the number of ms between reference pulses is 1/65.536 which is 0.01526ms.
the RPM is 1310720/1 = 1,310,720rpm (which obviously could never occur).

When N=500
the number of ms between reference pulses is 500/65.536 which is 7.6294ms.
the RPM is 1310720/500 =2,621rpm.

When N=1000
the number of ms between reference pulses is 1000/65.536 which is 15.2588ms.
the RPM is 1310720/1000 = 1310rpm.

Regards
Paul