I have a tiny tach on my 40hp Mercury and it works great. I know the RPM's while on the move and how many hours it has been running. I take it in at 100 hours for tune up. Sorry, no back light on mine. Got mine off Amazon.
Amazon.com: KEDSUM Hour meter Tachometer 2 & 4 Stroke Small Engine Spark For Boat Outboard Mercury: Automotive