I could not fit 38 mm tires under 45 mm VO fenders, but it depends on the bike. There needs to be enough room in the fork and stays to allow the fender to sit far enough from the rim, to have room for the tire. Ask your bike mechanic to verify it will fit before he does anything that will make the fenders not returnable. In my case, I used 32 mm tires instead and all was well.