Most rods have AFTM ratings as #3/4, #7/8 etc, indicating that lines of different ratings can be used. I was always under the impression the #3/4 meant bring suitable for a DT3 or a WF4. Is my theory correct?

What happens if you use higher or lower rated lines?

Thanks

No, your theory is not correct. This is one of the biggest and most persistent urban legends in fly fishing.

The dual line rating means either line will work and it's up to you to decide. There is no convention beyond that and it has nothing to do with line taper.

Grouse

This often used to be the case although when choosing a line I would recommend that you try as many on your rod as you possibly can - your fishing buddies may let you borrow theirs to help a little. Lines must fall between 2 grain weights to give them their aftm size so you may get 2 five weights for example, one of which may be at the lighter end of its band, the other at the heavier end, hence some lines may feel light or heavy for their size, yet still claim to be the same. I hope this helps.

to add - going heavier than the recommendation will result in the rod feeling 'loaded' with a shorter amount of line in the air, and with a lighter line more line will need to be in the air to get a similar 'feel'.

regards

andy

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I tend to go for the heavier of the two numbers. Makes for easier casting in my opinion.

From David Norwich's Blog who discusses this explicitly (Fly Rod Line Ratings and Fly Line weights):
http://www.davidnorwich.com/Davidsoc...lyFishing..htm

And appears to support your thought or theory. Depending on the rod, you should be able to handle less out of the tip on a higher rated line, or more on a lighter rated one. Some are so stiff they'll handle a few line weights higher without reducing head length outside the tip.

I fish a dt6 floating line on a 10' 6" 7/8 rod and it works fine for me.

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tried to post a link but it's not working. Lefty krey has a good article on lines for rods in which he recommends trying one if not two wts above and below rod ratings. I have an old 5wt RPL which casts a 3wt just as nicely.

You can use the line ratings as a guide but you may find that a lighter, or heavier, line is better suited to your rod.. and to you.

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probibally should mention that the aftm is based on the weight of first 30 feet of line, and with a bit more/less line out it weighs more/less so a #5 in effect becomes a #6 at 50 feet or so beyond the rod tip and vice versa.

recently i bought a new line. my rod is a 7wt so i tried a 6/7wt it was only ok, when i tried the 7/8 it was excelent so i would always try both now. The A.F.T.M only mesures the first 30 feet of the line and has an alowance of 8 grains - +