I just realized that my last tires on this bike were the GravelKing 1.75. I never took a caliper to them, but from what I've read, they're at least capable of measuring 43mm. My rim is the Pacenti PL23, and I'm afraid to say I've never actually measured the width of them, either. Anyway, the GravelKing 1.75's fit perfectly fine under the 50mm fenders. Perhaps my fender line isn't as close as I thought. Still, I may order the 55mm fenders just to be safe, although installing fenders is far from my favorite activity.