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, itīs always showing time...
Itīs not a precision instrument, to be sure, and would prefer a steady speed, it has a longish sample interval.
Iīve used small bits of magnets from old hard disks, theyīre very strong, fixed to the flywheel rim with
superglue. Iīve "calibrated" the readings with known rpmīs of rotating things, lathe chuck, 3-phase motors,
and they seem to agree quite well.
Itīs sometimes nice to know if an alteration to something in the engine has been in the right direction.
Iīve found doing it just by the seat of pants feeling is very unreliable.
