Hi Crazypj. Yes, tires vary a lot from their nominal sizes, which is why I've been so frustrated by Continental's rather screwy spec sheets. Fortunately, JCW has a 90/90 R 18 ClassicAttack and was able to measure the width for me. As it turns out, that tire is pretty darn close, at 91.5mm wide. One of the benefits of the Conti's radial construction is that radials don't gow much at speed, but I'm figuruing on leaving a good 1/2" on top, just in case. BTW, if you are reading this JCW, did you happen to measure the overall diameter of that tire?