What does a 7/0, 4/0, 8/0 etc translate into model talk? i.e. - is a 7/0 equivalent to a 50?, 80?, etc.
Well, I happen to know this... Two-part answer:
1) The 80, 130 designations and so on are just a semi-standardized table of sizes by reel manufacturers to categorize their sizes in the tableau of IGFA line classes. Usually a capacity of say, 800 yd of mono of a given line class.
In a general sort of comparative way, it works out to approximately:
4/0 = 30 lb class
6/0 = 50 lb class
9/0 = 80 lb class
12-14/0 = 130 lb class
18/0 = Unlimited, off the charts.
Things get confusing when people talk about say, a Tiagra 16 being a great 50-lb reel... They're talking about using superbraid lines and bumping up the ladder a bit.
2) What does /0 actually mean? A commonly held notion is that it is the letter O, short for offshore or ocean fishing reel. This is however not true. (isn't that sort of obvious if you have a 12/0 reel??) See the whole (long) story in an old thread at the reel historians club forum,
...and now you know.