My Hutchinson Globetrotter 700cX37 tires measure 39mm wide, my Zefal 45mm
fenders measure 44mm wide but the fender brackets offer 49mm of room. Thus
I have 5mm of room between each side of my tire and fender bracket. I
tried some wider tires and they did not work. From that I have concluded
that I need 5mm of clearance to allow for fender vibration, wheel out of
true, tire wobble, etc. I occasionally have to bend a fender stay if
something starts to rub, more than 5mm of clearance would be better but I
have used this fender tire combination for 8 years and am content with it.
If your fender does not come with a mud flap on the front, I am using the
lid of a large sized Folgers container, the plastic is soft and cut easily
with a scissors, but is stiff enough to hold shape and is sufficiently
durable to last.