I am replacing a FD on my Trek 2100. The seat tube outer diameter is 38.1 mm but when I look at FD sizes I see options for 28.6, 31.8 and 34.9. I guess I'm measuring the tube incorrectly.

Is there any way to tell for sure? I guess 34.9 mm is only 3.2 mm shy of my measurement and that could be a measuring error on my part - 3.2 mm is fairly small and the tape measure I was using isn't very compliant for measuring a small round tube.

Just replace it with another of what you've already got. Obviously it exists, right? So they do make them.

Exactly what is it that you've got?

FD's only come in the 3 sizes you have noted, the orginal one on you 2100 will have had the size printed / stamped on it. as DMF, replace with what you had.

Would suggest that you need to re-measure the clamp area diameter with a caliper, not tape measure, as it sounds like you have measured incorrectly.

It's metric equivalents of fractional tube outside diameters ..
divide by 25.4 and you get the decimal inch size.

1.125, 1.25, 1.375 inch.

Or measure circumference and divide by pi.

So, I removed my 5600 FD and found a 349 stamped on the inside of the clamp. I purchased a 6700 FD in 34.9 size and it too has the 349 stamped inside the clamp. But the clamp fixing bolt is so tight on the down tube that it just barely fits at all. I can "just" get the bolt started but even just started the FD won't move up or down or left or right with out a rubber mallet hit.

The fixing bolt on the old 5600 FD was the exact same size and it doesn't have this issue. I guess the newer clamp is thicker metal and that is preventing the bolt from reaching far enough to make this work better.

I do have it working and the FD works great, but really, there needs to be a longer clamp bolt.