motor RPM measure is very easy for Sanwa / Airtronics user. it have telemetry feature.
in fact Sanwa have infrared RPM sensor available for use with its RX461 RX462 already. but it range is very close about 1-3mm
in this clip I testing my infrared RPM sensor that increase detect range to 10-30mm
(using Sanwa MT4 - RX461)
YouTube >>
https://www.youtube.com/watch?v=fkV5SUt5MVI
see details about my infrared RPM sensor
http://mt4el.com/mt4-sensor/index.html