Standalone rev counter to calibrate my tacho?

[johnfm]

I need a standalone tacho - triggered off the coil or a plug wire - so I can get an accurate rev reading with which to calibrate my Smiths RVI tacho.

Any suggestions? Needs to be sub £30, as I could send the tacho off for calibrating for that money.

[johnfm]

Cheers

Found one on eBay for £8

Perfect. Takes a feed of the cylinder lead.