here is a site I found very useful with gear ratios and speedos, but there are a few basic things you need to know - trans gear ratio, (probably 1:1 final drive) rear axel ratio, speedo drive gear teeth count, tire diameter ( there are other sites that will calculate the tire diameter based on the tire size, just google 'tire diameter calculator' try a couple different sites to verify the calculation) plug it all in and it spits out the speedo driven gear tooth count....It's easy you see, but really just take it one step at a time and you'll get there.
http://www.transmissioncenter.net/sp...n_______va.htm