Bicycle Mechanics - Fender Sizing

I ordered fenders from Trek for my 2008 (? I think) FX 7.2 with 700 X 35 tires. In looking on the Trek website I noticed there were several sizes, and I "assumed" (yes, I know what happens to people who assume) that a fender characterized as 700 X 35 would fit a tire of that size. But, so as not to be presumptious, I called rather than order on-line, and the idiot taking my order assured me that my "assumption" was indeed correct. But when they came, the fenders are the same size as the tire. No way they will fit. What bike measurements are used to properly size fenders?

go for 10-12 mm over the size the tire. 45mm at least.

Ignore the tire size and get the widest fenders that will fit between the fork legs and seat stays. The 10-12 mm wider than the tire recommendation is the minimum if they will fit.

Thanks Bio, that makes sense, but so did ordering by tire size. LOL

Thanks Bio, that makes sense, but so did ordering by tire size. LOL

Well, it was obvious to me you want the fender to be wider than the tyre...

Yes, operator, I concur, the fenders should be wider than the tire, but since this is my first fender order, and since the Trek telephone answering person agreed, I thought a 700 x 35 fender was made to fit over a 700 x35 tire, and we were both wrong. No matter, I sent them back to Trek, and they are sending the 700 x 45, and shipping free. I only had to pay return shipping, but $12.00 worth of knowledge is cheap.

This is the kind of thing you avoid if you go to the LBS

This is the kind of thing you avoid if you go to the LBS

What... he said.