Nice link Mystic. Much easier to read than the jpg I had been posting to answer this question.
2 points to make:
Speed rating. The charts decipher the tire markings and explain the speeds involved. None of the charts I've seen fully explain that these speeds are "sustained speeds". Ex; An H rated tire is rated for sustained speeds of 130mph. Now you might say:what does this mean to me- I never break 100. The higher rating a tire has, the cooler it will be at 55mph. The cooler it is, the more miles you will get out of it. Most "good" tires start at "H". When I started in the M/C business in '85, the general rule was: H tires were touring, V tires were sport. Then bikes requiring radials changed that.
Tire size. This subject has been beaten to death, yet it still lives on. Car tires are flat. You increase size, you will have a larger contact patch. Motorcycle tires are curved. You increase size, you squeeze it on your stock rim and put more curvature on the tread face giving you less rubber in contact with the ground. Our bikes came with inch sized tires, so a metric tire is already a little larger. A compromise we have to make to get decent tires. But the idea that "if a little bigger is OK, than a lot bigger is better" is wrong. Fat tires look cool, no question. I had a 750K4 that called for a 4.00, had a 130(5.25 equivalent) when I first got it. It would "fall" when starting even the slightest turn. It also would lock up fairly easily. It didn't do that after I put the correct size tire on.