The numbers preceding the slash indicate the distance, in millimeters, across the widest point of the tyre—called section width (245mm, or 9.65 inches, in this case)—when mounted on a wheel of specified width. Often listed before this number are letters that loosely signify the kind of duty for which the tyre was designed: “P” stands for “p-metric” and is generally used on passenger cars, “LT” indicates light-truck duty, and “T” is for a temporary spare.
This two-digit number is the aspect ratio, or profile, of the sidewall. This tyre’s sidewall height is 40 percent of the tyre’s width, which equates to 98mm, or 3.86 inches. The lower the number, the shorter the sidewall.
This letter indicates radial tyre construction; nearly all tyres sold today are of this variety. Other constructions are “D” for bias-ply tyres and “B” for belted. A preceding “Z” is simply a reference to an outdated and vague speed rating of more than 240 km/h, or 149 mph (the specific rating can be found in the service description)
This number indicates the diameter of the wheel on which the tyre should be mounted, generally in inches. These are usually whole numbers but can also be half-inch increments.