I'll post a speedo calc. TCI had a great one and of course took it off their website. This one is okay even though its a Ford site.
I dont know if you have a 7 or 8 tooth drive gear, and you cant just count the 'threads.' You have to lay them flat on their face to count them.
So If it was me, I would calculate for both and then pick a driven gear thats between both answers.
You'll need your tire height, rear axle ratio and 7 or 8 tooth for a drive gear