Would it matter much if I used 6wt. line on a 7 wt. rod? I can’t see why it would create any big problems.
Opinions appreciated.
A DT 6 and a WF 7 will work pretty much the same. if you have some experience, you should be able to adjust your stroke down to even a DT5.
lastchance -
Depending on the rod and the line, it could make a HUGE difference, or none at all.
I just went through that overlining / underlining experience buying a new Sage. Started with a 6 wt TCR ( technical casting rod ) overlined with a weight forward 7 wt line. After getting the hang of the TCR, its a super fast rod and required me to really concentrate on a different casting stroke, the line and rod worked very well together.
Then I got a Sage Fli 7 wt which I demo’d for a day with a standard weight forward 7 wt line. It’s a fairly fast action rod, and felt good with the standard line. When I bought it, we put a Rio Grand line on it, thinking the Fli could handle the heavier ( by about half a line weight ) line. WRONG. The line was way too much for the rod - bogged it down terribly. Went back and put a Rio Selective Trout on the rod, and it works beautifully, once you get enough line out ( 20’ plus ) to load the rod.
Generally, overline a rod to load it quicker close in, and underline a rod to cast longer. Depending on the action of your 7 wt rod, it might feel like a broom stick with a 6 wt line on it, until you get a whole bunch of line out to load it. Or, if it is a slow action, it might handle a DT 6 wt just fine.
I’m sure others have some different experiences, and maybe some very different thoughts than these.
John
just as john says—it depends on the person—thier experience and the rod/line combination —sometimes a rod has has a totally different feel with a lighter line—
i cast the 7wt sage tcr with a 6 rio grande line and i could pound tacks with it out to about 55-60 ft —it seemed to run out of gas for me after that whereas the zaxis 6wt with the same line did the same in close but it did well beyond–this tells me about the range of power and whether im overloading/underloading the rods not that i would fish at that distance.
I agree totally Shorthaul. John Scott has put in easy terms.
If you could try different lines, it could open some eyes.
I use a 7 wt. Windcutter II on my BIIx 8 wt. and it is awesome.
Any accomplished caster (fly shop owner?) can pitch good loops with just about any rod and line combination, he has to to sell them. But, fishing one all day is not the same. We all develop our own ‘style’ or a method of casting that we find comfortable. Some of us tend to be wrist casters others push-pull piston type. Some utilize the tip of the rod more, others load the rod deeper. Basically, tip action rods or progressive action. Now add over/under lining and you have an infinite amount of combinations of, " I like this, I hate those, this sucks, you’re nuts, etc". IMHO the TCR is a tip caster, the Z-axis, progressive. I love them both; have both and fish both. (so does my wife 
)
Can you explain the why to the above statement?
When I think of underlining a rod, I think of someone who casts great distances. The greater amount of line in the air loads the rod more than the stand wt line at short distances. I think more people over line, since most people don’t cast great distance and this overlinning loads the rod better for the 25’ or 30’ cast. If you are carrying 60’ or more of line in the air underlinning one size should cause no problems.
The above doesn’t take into account different rod actions at all and is just a general statement with all other things being equal.
I like this statement. It summs things up so well, given there are so many variables.
Clay, I’m with you too. I hardly ever have a need for distances over 55ft or so and that only in warm water with muskie flies etc.
Trout stuff needs to be kept to gentle, precise close presentations for my little spring creeks rivers.
What I’m hoping the orig. poster doesn’t get too caught up in are the technicalities of all this stuff. He needs to stick to the basics and get good up close. Forget those longish casts for quite some time. They’ll come in time. Too soon for my feeling.
Man I’d love to cast alongide of some of you guys. I’d learn a lot I"m sure.
Jeremy.
Well, I’ll try it out and let you how it works (feels) to me. I would think if I was using it as a nymphing line it probably wouldn’t matter much. Casting a dry fly, I’m sure, would be noticeably different. I guess the flex of the rod would have something to do with it also.
I’m not a great dry fly caster, although I catch my share of fish during a hatch. I’d never enter a competition as a caster for sure. Most of my dry fly fishing is done within 40 to 50 feet of me.
I’m going to the Grand River on Sunday and I wanted a larger rod to throw some streamers around but I don’t have a 7X line to match my 7WT. rod.
Just wondering what more experienced (professionals) fly fishers think.
I was reminded of a couple of things while rereading a fly fishing book over the weekend.
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The actual difference between one weight of flyline and the next is measured in hundredths of an ounce. Many of us aren’t good enough casters to even notice the difference.
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While line rating is a standard measurement using the actual weight of a measured portion of line, the weight rating of a fly rod is completely subjective and reflects the manufacturer/rod builders intent based on the way the rod was designed to be used. Unfortunately, there are few manufacturers that tell us under what conditions the rods was designed to be used. Why is that?
You’re right, Rick. I had forgotten about that. It’s like the difference in stiffness and weight between singlefoot and doublefoot guides. Some say the doubles are heavier and make the rod a bit stiffer than singlefoot guides, but you have to be a real expert to notice the difference.
I love hearing other opinions because that’s how we learn.
Thanks
Sounds like you already have some 6 wt line and a 7 wt rod. Go cast that bad boy in the lawn brother and see if you want to fish it all day like that. 
The answer will come to you quickly. Let us know how it feels to you.
Rick
I use my 5wt cortland camo intermediate on my 6wy St. croix legend ultra, casts just swell. Some rods will cast different line sizes than what they were designed with different casting strokes and practice.
Well said, Rick B. The only thing I would add is that different types of tapers will have a different effect on any rod. A “Rocket” taper will load different from a “Standard” weight forward taper. A double taper will load differently from a flat line. Sink tips will pick up off of the water different from floating lines. Leader and tippet length, taper and weight probably factor in there somewhere along with wind resistance/weight of the fly but I am not that good of a fly flinger yet.
IMHO, the best thing to do is experiment with different weights and tapers with your rod and find out what works best for you. Once that is done please share your findings with us. As long as it works for you then what I say really doesn’t matter. I applaud thinking outside of the box.
While your at it treat your lines and guides with ZipCast. You will be amazed at the difference.
Nighthawk: I was pretty much thinking that it couldn’t make that much difference especially if I’m just using it to nymph or throw streamers. I think I will try to cast some dry flies just to see how it feels.
Thanks
If I am showing a non-serious beginner how to cast I will over-line them every chance I get. They will cast acceptably well and enjoy themselves plenty and do it faster. My wife overlines her pink Sage by two weights to find a spot she enjoys casting.
If I am showing someone that really wants to learn how to cast I will start them with the stiffest, fastest rod possible and underline it, maybe even two sizes. Force them to understand the critical nature of timing when forgiveness is hard to come by. When they start laying out long casts with critical timing the rest is easy.
That said, give me a forgiving rod these days. I can still cast farther than I ever need and it makes fishing more pleasant.
I am far taller than average with a wingspan built to shame apes. I learned to fly fish on salmon with long rods and heavy flies and developed some “unorthodox” style in my casting. A very well-known, now late, fly fisherman, author and all sorts of stuff once remarked (I was in my 20s) “Son, you cast farther with less grace than anyone I have ever seen!” There is more than one way to get a job done…
art
As has been mentioned, there are a ton of variables involved. But basically, a manufacturer will build a rod to have a certain action with a certain weight line. Using a heavier line will slow down that action. Using a lighter line will speed up the action. If your 7 wt rod already has a fast action, underlining it may make it unusably fast. If it has a more moderate action, it might make it just right.