Cliff,
There was a link on here some time ago that would take you to a site with a calculator for this. But the teeth on the driven gear ( the one on the end of your cable ) are worth around 4 miles per tooth. If you have your's out and know about how slow your speedometer is you should be able to go to a local Ford dealer and they should be able to tell you which gear you need. Also make sure they get the right twist as some cars had a left twist while others were right but I don't know which ones had which. If you know how slow yours is reading then decreasing the number of teeth will speed it up approximately 4 miles per hour. Example ( With your tires and gear ratio if you are reading 12 miles per slow, decreasing the gear by three teeth will put you right at correct. But it may not be exact. ) Hope this helps.
Ron
