Breaking down 255/45R18
Take a size like 255/45R18. Each part encodes a separate measurement: 255 is the section width in millimetres, 45 is the aspect ratio as a percentage, R means radial construction, and 18 is the wheel diameter in inches the tire is built for. Read left to right and it's just four pieces of information stacked together with no spaces. Nearly every passenger and light-truck tire sold today follows this exact format.
Section width: the first number
The 255 is the tire's section width, the distance across the tire at its widest point (sidewall to sidewall), measured in millimetres with the tire mounted on its specified rim width and inflated. It is not the same as tread width, which is usually somewhat narrower. Section width is the number that has to work with your wheel width: a tire is only designed to sit correctly on a fairly narrow range of rim widths, typically plus or minus about an inch from its ideal, so pairing a 255-width tire with a way-too-narrow or way-too-wide wheel distorts the sidewall profile and hurts handling.
Aspect ratio: the second number
The 45 is the aspect ratio, the sidewall height expressed as a percentage of the section width. A 45 aspect ratio means the sidewall is 45% of 255mm, or about 114.75mm tall. Lower numbers mean a shorter, stiffer sidewall (sportier handling, harsher ride, more prone to pothole damage); higher numbers mean a taller sidewall (more cushioning, more sidewall flex, a softer ride). This is the number that does the work in plus-sizing: as wheel diameter goes up, aspect ratio has to drop to keep overall diameter constant. See the plus-sizing-wheels guide for how that trade-off plays out in practice.
Construction, diameter, load index, and speed rating
The R stands for radial, the standard construction method for essentially every tire made today (older bias-ply construction is marked with a "B" or "D" and is now rare outside specialty and trailer tires). The 18 that follows is the wheel diameter in inches the tire is designed to mount on, matching your wheel's diameter exactly; there's no flexibility here. After the size, you'll often see a load index and speed rating together, like "94V." The load index (94) is a code that maps to a maximum weight per tire, roughly 1,477 lbs for a 94; higher numbers mean higher capacity. The speed rating (V) is a letter representing the tire's maximum sustained speed rating, with V standing for 149 mph. These matter most for trucks carrying loads and for matching a replacement tire's capability to the vehicle's original spec; going below the factory load index isn't recommended even if the tire size otherwise fits.
How tire size ties back to your wheel
Every part of the tire size interacts with your wheel choice. Section width needs to suit your wheel width (check the tire manufacturer's recommended rim width range, usually printed in their fitment guide). Wheel diameter has to match exactly, no rounding. And aspect ratio combined with section width determines the tire's overall diameter, which is what keeps your speedometer accurate and your fenders clear when you change wheel size. If you're shopping for new wheels and want to know what tire size pairs with them, our fitment tool at /tools/wheel-calculator shows compatible tire sizes for every wheel width and diameter combination we carry.
Reading sizes on AlloyHaus
Every wheel listing on AlloyHaus shows the tire sizes that are actually recommended for that wheel width, so you're not left cross-referencing rim-width charts on your own. Once you know how to read 255/45R18, every other size on every other tire follows the same four-part logic.
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