Hi, Same as 10 & 12 in backspacing, inward is limited by the uprights and linkarms.
14in is just 2 inches more outward from center.
Not many tyres to chose from though in 15in.
12 in measured from inside the bead and using avon 295 you will need to fit inner tubes into the tyres to get them to ‘expand’ overnight. Then stretch them on. Its a 3 person job to fit.
I have a set of Dunlop Racing tyres thats 14x15 thats going onto 14in rims for a 917K build.
morten