As long as I’m giving my ratio lecture, I’ll add part 2: what it means to your yarn (part 1, above was “what it means to your wheel”). You can infer it, but I’ll be explicit because it’s not so obvious when you’re already trying to figure out what your wheel is doing for you.
Your ratio is telling you not just how many times the flyer is spinning, but how many twists are entering your yarn for every treadle. Now, this doesn’t provide you with useful information until you consider exactly how you are spinning your yarn.
For a very broad but possible example, if you are spinning with a wheel set at a ratio of 5:1, and every time you treadle you advance your yarn forward exactly one inch using a short forward draw, allowing it to feed into the orifice at that same 1”/treadle rate, you will have 5 twists/inch of yarn. Now I don’t often draw forward exactly one inch, but I do sometimes spin very bulky yarns with a short forward unsmoothed draw (don’t laugh, it’s lofty that way, and even), at something like 2”/draw and attentively drawing with each treadle (thank you for your class, Maggie Casey), so if I have my wheel set at 6:1, I know that I will get 3 tpi in my singles fairly consistently.
At the other end of the spectrum, if I have my wheel set at 17:1 and I use a long backwards woolen-type draw and draft back 18” at a time like I often do, and it takes me 2 treadles to draft and even out my draw of fiber, then I only have around 2 tpi at that point--nowhere near enough to make a stable fine single. Not only that, but every treadle is only adding about 1 tpi for that length of fiber. This is why usually when you are spinning in this style you are taught that after evening out your draw, you must treadle several times without allowing the fiber to draw into the orifice; you need to build up adequate twist to make a stable yarn.
Now is there going to be a huge difference to your yarn between 5:1 and 6:1? You can probably produce an identical yarn with a little work. One or the other will probably feel more natural to you, though, in producing any one particular yarn. Your grist, your fiber type itself, your desired look and feel, your plans for plying and for use; all of these will help dictate what ratio you ideally will use for any particular yarn you spin.
About the extremes; there are wheels which spin in the 3:1 neighborhood, and a very few which will work at 40:1. There are only a tiny handful which will operate nicely at either of these extremes, though. The ones which work at very low ratios are often specifically designed to spin bulky or art yarns; they have extra large orifices or no orifice at all; they have extra large bobbins available; they have space on the wheel and flyer for a reasonable amount of bulky yarn. Very high ratios have their own issues, and the biggest one is what I mentioned above; the basic physics which requires more force to produce more energy to spin the flyer that fast. In some wheels, this makes treadling such a chore that while it’s possible, it’s just not pleasant enough to be useful. In a very few like the Schacht, the wheel is so well engineered that treadling even at those high ratios is not onerous. Then there are wheels like the Spinolution Bee, which solve the problem by using a different track for the bands to spread out the physical work and a different treadling motion which while easier to the muscles seems less efficient physically to me (just opinion; I can tell you that if you enjoy the treadling on the Bee, treadling at high ratios is not at all difficult as it is on many other wheels, but I would never tell anyone to buy that wheel without sitting at it first; personally, I like the Bee a lot and it’s a wheel I would consider getting since it can spin at higher ratios than my Fricke, which few others can). It’s not difficult to engineer the whorls that allow those ratios from say 13:1 to 20:1 and above; most wheels just aren’t comfortable to treadle for beginners at those moderately high ratios, and I think that’s the primary reason why most wheels don’t go that high without add-ons.
For those higher speeds of twist transmission, electric spinners, charkhas, and supported spindles are all good alternatives too. Charkhas (and other spindle wheels) and supported spindles like taklis or Russian or phang spindles are tools specifically designed to put a lot of twist quickly into delicate, fine fibers like cotton or goat or camel downs. I comfortably spin lace yarns from short (2-4”) fibers on my wheel up to about 4500 yds/lb, using a 17:1 ratio to spin the singles and a 12:1 to ply. Above that fineness I typically would choose a supported spindle unless I was using a strong, longer wool or some other variable factor dictated a preference for using the wheel.