I have 55mm BA's on some 1995 MTB rims and they measure 54mm. I have 60mm BA's on Dyad rims and I haven't measured them but they are HUGE. I do a lot of reading on bicycle forums and websites and the 50mm BA's seem to consistently measure closer to 45mm. The LBS has a bike with 50mm Big Bens and they are tiny, even compared to my 55's. My experience is you will get fat tires with 55 or 60mm BA's but you may not get fat tires with 50mm BA's.