OK I see that if the pitman arm on a box moves through 90 degrees - lock to lock and it takes 4 turns on a GT box to do this then to move the pitman arm 360 degrees ( 1 turn ) it would take 16 turns = 16 :1, the gear ratio.
I have only seen the spec of 16:1 given.
Do you know how the overall steering ratio of 21.7 :1 is calculated ?
I am curious thats all 
Would it be how many turns on the steering wheel to move a road wheel 360 degrees ( if it could ?)
To eloborate on a discussion from another thread, my findings regarding this subject are:
1. Steering gear ratio: Ratio of revolutions between input shaft and output shaft.
2. Overall steering ratio: Ratio of the turn angle on the steering wheel to the turn angle on the spindles = steering gear ratio + gearing effect (leverage) from the steering linkage, e.g. a 22-degree turn on the steering wheel (ex slack!) results in a one degree movement of the spindles.
One source of relevant Mustang specs can be found here, including the 21.7 : 1 number:
http://mustangtek.com/Library2/MustangaCompleteGuide.htmlGo to page 22.
