The middle number on tires refers to the tires diameter, which is the rim diameter of the wheel that the tire will fit. This number is typically the last number in the series of numbers and letters on the tires sidewall. For example, in the tire size P215/65 R15 95H, the "15" refers to the rim diameter of 15 inches. Its important to note that the numbers on a tires sidewall also indicate other information, such as the tires width, aspect ratio, construction type, load index, and speed rating. Understanding these numbers can help you choose the right tire for your vehicle and driving needs.