Well, You can usually put a larger tire on the standard rim, but there is a limit to how large you can go with a certain rim width. A good tire dealer will steer you right as to how large a tire you can SAFELY install on your current rims. The WD45 rims are probably a bit wider than the WD rims, so if you can't get the tire you want on your current rims, you could look around for a set of WD45 rims, or go buy new ones (large $$$). As for the numbers on the tire, they refer to the width of the tire at it's widest point (called section width) when mounted on a specific width rim. The larger the first number, the wider the tire.