No question about it, an 8-weight rod with an 8-wgt. line beats a 6-wgt. rod with a 6-wgt. line any day in wind fighting. Put the 8-wgt line on the 6-wgt. rod and it does more than put more weight on the rod broadening the loop, it overloads the rod! A good way to think of it this: for every five feet of line you aerialize beyond the rod tip, you effectively add another line weight. So...... if you carry 45 feet of line into the backcast, you effectively are casting an 11-weight line, not the 8-weight you started with. What happens when you do this? With a 6-weight rod - now overloaded by 5 line weights - expect the cast to collapse or in the worse case, the rod to explode. (And as the shards of graphite settle around you and the Ancient Fish Gods laugh ... as you resolve never to overline again.) There is some variability in this 5-feet rule: it's the length of the belly beyond the first 30-feet, but 5 is good enough for me.
What seems so logical, isn't! Overlining is a problem I have more trouble with than anything else in teaching the fly cast, particularily if the fly fisher has experience. I truly believe it's a holdover from baitcasting of spinning. When Mr. Wind begins to blow, what did we do? Add more weight to the end of the line and sure enough we could cast further.
How many times have you driven down a highway against a headwind only to find that your gas milage has gone to hell and your power seems diminished ... the reason, of course, is the added resistance of the wind against the front of your car. Your 6-weight rod has the same problem ... not enough horses under the hood to negate Mr. Wind's effect.
I don't know whether it will help further discussion, but I am adding an extract from my book, "Fly Fishing for the Rest of Us," and the discussion of fly lines... I think the key to the subject of line weight rests in understanding the AFTMA standards ...
(QUOTE) "A Summary of Key Points. At this point in our ongoing discussion, it’s appropriate to summarize and reinforce several points worthy of your consideration as you go about selecting a fly line.
• Keep in mind that the only thing constant in the gentle art of fly fishing is the AFTMA standard for fly line weights. Consequently, only fly lines can be discussed objectively; everything else, including the rods, is subjective. This implies choosing the line weight should be your priority when setting up a new fly fishing rig. It’s fairly simple to select the right line weight for the task if you identify: (1) the primary species you plan to pursue; (2) the type waters you plan to fish; (3) the climatology of the region; and (4) the size of the flies, streamers or bugs you intend to throw.
• Choose a larger line weight to throw a larger fly. It’s the mass of the fly line that pulls along the mass of the fly. Select the right gun (line) to do the job. The same is true for the rod. Don’t let the esthetics of a slender lovely little 3-weight rod throw you into a tizzy when a 7-weight is what you need. Of course, if you are into buying lots of fly rods, be my guest.
• Don’t overline! Overlining is a “phenomenon” that occurs when a fly fisher selects a heavier line - say a 7-weight floater - to use on a 6-weight rod. Dummies do that because some jackass said, “it will cast better.” It won’t! While it’s possible for a rod to be rated incorrectly, it is indeed rare. The notion of overlining is “crapola” propagated by either those (a) who cannot cast, (b) those who do not know much about fly fishing, or the cons who want you to think you are a "fly caster" within the first 5 minutes. I detest cons! The fact that he or she may sell fly fishing equipment means zilch in terms of creditability! There is a single exception to this statement -- it’s “short-range” casting to targets less than 30 feet away. In this instance, it’s true that a 7-weight floater will load a 6-weight rod to cast 30 feet or less. However, if you learn to tip cast, overlining for this specific situation becomes unnecessary.
• If you happen to be a Doubting Thomas, consider this -- 30 feet of fly line weighing 140 grains outside the rod tip is a 5-weight line by definition in the AFTMA standards. Add another 5 feet to the length, and what do you have? The 35-feet of 5-weight line now weighs the equivalent of a 6-weight. The simple fact is adding 5 feet of line, in effect, adds one line weight. Said another way, aerializing 35 feet of 5-weight line on the backcast is the equivalent of lifting 30-feet of 6-weight line into the air! No question about it -- you need to understand the idiosyncrasies of line weights and how they can work for or against you. In the heavier weights - 8, 9 and 10 - I often recommend using a rod weighted one weight above the line weight to be fished. For now, perhaps this example will suffice: When I demonstrate “Long Casting,” I use a 10-weight line on a “light” 12-weight rod called WindTamer. (The term WindTamer is mine having first published it in May, 1995.) And yes, you read it correctly -- the rod is two line weights heavier than the line; and no, I am not embellishing facts. As an old Druid, I speak only the truth! More on this subject later.
• Most rods will cast more than one line weight. Some might do it better than others, but normally you can count on at least two line weights for almost any rod down, but nor necessarily up. You might even find your 7-weight will cast a 3 or 4-weight line well enough to catch fish. This usually works best using a technique I call tip casting. True, you may not be able to perfectly emulate what can be done with a 3-weight system, but you will catch fish, have fun and save money, if that’s important."