This might be common knowledge
http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/I built one
Cost 7.50 euro, from Lidl. Can be had cheaper from ebay. Doubles as a clock, it´s always showing time...
It´s not a precision instrument, to be sure, and would prefer a steady speed, it has a longish sample interval.
I´ve used small bits of magnets from old hard disks, they´re very strong, fixed to the flywheel rim with
superglue. I´ve "calibrated" the readings with known rpm´s of rotating things, lathe chuck, 3-phase motors,
and they seem to agree quite well.
It´s sometimes nice to know if an alteration to something in the engine has been in the right direction.
I´ve found doing it just by the seat of pants feeling is very unreliable.
