Originally Posted by flyflingerandy
The reason the 6wt worked better than the 5wt, would have to be because all fly rods (except Orvis and Winston WT 2-5wt) are rated for DT lines, not WF.
This is an old myth. It may have been true in the past, but is not now. When you consider the casting distance for most trout fishing 20-45 feet, you will see why. The AFTMA weight rating for a flyline is based on the first 30 feet of a flyline excluding the tip, which is typically about a foot in length. If you are using a 8 foot rod with 30 feet of line out of the rod's tiptop and a 7-1/2 foot leader this adds up to a 46-1/2 foot cast if the line straightened completely. Assuming 1-1/2 feet of slack, you get around a 45 foot cast (= 8ft +1 ft +30ft +7.5ft). A DT line and a WF line with 30 foot belly sections, the same tapers, and the same belly thickness/density will have identical performance out to about 45 feet. I have had this discussion before on other. Bruce Richards of Scientific Anglers posted in the thread confirming this. He also wrote a short article about it.
Most trout rods are designed to optimally cast about 30 feet give or take 5-10 feet) of line. As you get up toward the saltwater line weights, the casts are typically longer and the rods are designed to carry longer lengths of line to accommodate these longer distances, but this doesn't generally apply to trout weight rods (especially 4wt and under). Thus if you are usually casting shorter distances than these rods are designed for, you may want to go up a line weight. If you typically cast longer distances than the rods are designed for, you may want to up a line weight. If you typically use one or two line weights heavier for every casting situation at nominal distances (20-40 feet), you probably prefer a slower action rod than you are using.