Matched line weights

[tailingloop]

In MY humble opinion, ABSOLUTELY NOTHING. You'd have to be some kind of a bionic robot to notice the difference. In a controlled BLIND study, using two SAME wight and length rods, there is ( almost) NO way I'd be able to discern which rod was lined wirth which line. This from my own experience adding a bit of "WHO CARES ?". Two or three line weights deviation would/could beg some adjustment but one ???????
Mark

The ideal double bind test is two rods of same make and model with the same model reel mounted using the same model line that is one weight difference. And I agree the the effects of going up or down a line size quite often will not be discernible, especially at the 30 ft mark that is used to establish line weight.

My own personal experience is that I am much more likely to notice a difference with ultralight or heavier weight rods than the 4-6 wt rods commonly for trout fishing. So I find going up up or down a line size is more notable to with a Sage TXL 00 wt rod or Sage 10wt than a typical 5wt rod. I have half a dozen 5wt rods and am hard pressed to notice much difference with 5 of those rods.

I also find that with models of rods I am more likely to notice a difference than with others especially with slower action glass rods. That one 5wt that is more sensitive to me is built on a slow glass blank

Also, differences in taper ( how weight is distributed along the length of line being cast) of the fly line can also have an effect that is more noticeable to me than going up or down a size, especially as the length of line being cast is changed