Props to the boys at LMRS for this easy to use speedo gear calculator. Put in a few variables and it shows you what tooth gear is needed. Even shows a picture.
http://www.latemodelrestoration.com...ail&utm_term=0_cb1e7fdfb3-4f8824a663-50847045
http://www.latemodelrestoration.com...ail&utm_term=0_cb1e7fdfb3-4f8824a663-50847045