Hi guys!

So, my 2014 Honda Shadow Phantom 750 (as any Honda Shadow, really) comes without a tach. I’m considering to buy one. I think on my previous one it might have been a Biker’s Choice tach, and so I’m even considering the same one, as I was happy with it. But checking around, it seems I need to know beforehand the drive gear ratio in order to get proper measuring. How can I find out this on my bike?

Thanks!