I have a mini tubbed '69 RR. The tubs have been widened 2 " so are about 3/4" shy of the frame rails. I have 16" from the inner tub to the wheel lip but of course the outer tub curves in towards the top. I figure any tire with a 14" cross section will be safe (maybe even 15"). The issue is finding the proper height tire - in my case I need a 28" or 28.5" tall tire to fit a 15" dia. wheel.. Any taller will rub the front wheel opening. There are a lot more tire choices if you want to step up to larger diameter wheels like 16 to 20 inches but I'm not after the "touring" look either.