Tires with a 305 or 315 section width are typically low-profile tires to go on a wide rim or oversize off-road tires. Tire sizes step up in 10 millimeter increments, so a 315 tire is one size larger than a 305 tire.
The "305" or "315" of a tire size is the section width, or the width of the tire at its widest point in millimeters. A 315 is 10 millimeters wider than a 305 tire. There are 25.4 millimeters to an inch, so the 315 tire is 12.4 inches wide at its widest point, 0.4 inch wider than a 305 width tire.
The height of a tire can be calculated from the section width and the aspect ratio. For example, a tire size 305/60R18 has an aspect ratio of 60 percent. The aspect ratio is multiplied times the section width to calculate the section height of a tire. A 10 millimeter width difference will be a 6 millimeter height difference on the example tire. Multiply the difference times two and divide by 25.4 to get the diameter difference. For a 60 percent aspect ratio, a 315 tire is 0.48 inches taller than a 305 tire.
A 315 section width tire will fit on the same wheels as a 305 tire. Tires should be the same size on the same axle. You may be able to replace a 305 tire with a lower aspect ratio 315 tire and still have the same or lower tire height.
