Mike, I have some Guy FBAX trucks!!! I would like an original copy of a book! And they are 9.00x20 tyres on the later ones. It appears only the very early FBAX in the late 20's early 30's had the 9.00x22 tyres, at which I do not believe any to be in existance anymore.....
It is interesting seeing all the parts on the FBAX, it has FBAX part numbers cast into a lot of the housings including the engine. The foot brake is mechanical with servo assist to the middle axle only, and the hand brake is mechanical to the rear axle. No brakes on the front axle. And yes, there are 2 worm drive diffs in the back.
Also Peter, if you come across any FBAX photos in the archives, please post them as photos of them are VERY VERY rare.
Cheers
David