Ratio of Mechanical Tach

Alright here's the deal. I have an '83 GPZ 550 with an '81 GPZ 550 motor. The motor has a pickup for a mechanical tach but the display uses an electrical tach. The tach is properly wired but something is wrong with the actual mechanism. So I wanted to switch to an aftermarket mechanical tach and just modify the display panel. Was wondering what ration I need and it this would work: cgi.ebay.com/ebaymotors/Motorcycle-Chrom...dZ1QQsspagenameZWD1V ?

Thanks,
Nate

Not really an answer but here is a thread that shows a link to some from Old Bike Barn that look just like the auction one.

kzrider.com/component/option,com_joomlab...atid,4/id,9920/#9920

You can figure out the ratio just by watching the tach drive while turning the engine over by hand.

You should also be able to use an electronic tach, if you want.