I picked up a tachometer from a model shop, designed to measure model aircraft engines. It's not incredibly accurate at the sort of revs. we are used to but it is good to the nearest 100 rpm. It's an optical device that would normally sense the passing of a two or three bladed propeller. It therefore is reasonably proficient at sensing flywheel spokes or lines drawn on flywheel rims with the application of the appropriate conversion factor.
Not the best solution but cheap and reasonably capable.
For lower revving things (< 100 rpm), I use a clock/watch timer but that is another story and not at all cheap!
Regards
Mark
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