Sorry, can't find a listing.
I'd take a guess & suggest an 18 tooth gear.
There used to be about 6 different gears available (how many teeth).
Try the Conventry Group http://cglonline.com.au/coventry/
look under Automotive and get a phone number for your state and order one.
Try it out & get someone to follow you while your doing 60. If they indicate your doing a higher speed than 60 try a bigger gear (more teeth), and visa versa for lower speeds.