Decode a tire size code (like 205/55R16) into its actual sidewall height, overall diameter, and circumference, and compare it against a second tire size.
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A tire size code such as 205/55R16 packs three key measurements into a compact standardized format: the section width in millimeters (205), the aspect ratio as a percentage of that width (55%, meaning the sidewall height equals 55% of 205mm), and the wheel or rim diameter in inches (16). This tool decodes those three numbers into the tire's actual physical dimensions: the sidewall height, the total overall diameter (rim diameter plus two sidewall heights, since the sidewall appears above and below the rim when mounted), and the circumference, calculated using pi multiplied by the diameter, which in turn determines exactly how many times the tire rotates per mile or kilometer traveled — a figure that matters directly for anything sensitive to actual rolling distance, such as speedometer and odometer calibration, both of which are calibrated by the manufacturer assuming the original factory tire size. This is why the difference matters when switching to a non-stock tire size: a larger overall diameter rotates fewer times to cover the same distance, causing the speedometer to under-read actual speed, while a smaller tire has the opposite effect. Tire manufacturers generally recommend keeping the diameter change within about 3% of the original size to avoid throwing off speedometer accuracy and systems like anti-lock braking, which also rely on wheel-rotation speed readings.
The string of numbers and letters printed on a tire's sidewall — something like 205/55R16 — looks cryptic at first glance, but it's actually a tightly standardized international code that packs three essential measurements into a compact format, allowing any tire shop worldwide to understand exactly what dimensions a tire has without needing to physically measure it.
The first number (205 in this example) is the section width in millimeters — the tire's width measured from sidewall to sidewall at its widest point. The second number after the slash (55) is the aspect ratio, expressed as a percentage of that section width, which tells you the sidewall height: a 55 aspect ratio on a 205mm-wide tire means the sidewall stands 55% of 205mm tall, or about 112.75mm. The letter R indicates radial construction (the standard for essentially all modern passenger tires), and the final number (16) is the wheel or rim diameter in inches that the tire is designed to fit.
From these three numbers, several other useful dimensions can be calculated. The overall tire diameter equals the rim diameter (converted to a consistent unit) plus two sidewall heights, since the sidewall wraps around the rim and contributes to the tire's height both above and below the rim's edge. From the overall diameter, the tire's circumference follows directly using the standard formula, circumference equals pi multiplied by diameter.
Circumference matters practically because it determines exactly how much ground distance one full tire rotation covers, which in turn determines how many rotations occur per mile or kilometer of travel — a number your vehicle's speedometer and odometer are calibrated against, based on the factory-original tire size. Fitting a noticeably different tire size changes the actual rotations-per-mile figure without the speedometer knowing, which is why a plus-sized or significantly different tire fitment can cause your speedometer to read incorrectly.
This is why tire shops and enthusiasts pay close attention to matching overall diameter when changing tire width or wheel diameter for cosmetic or performance reasons — keeping overall diameter close to the factory original, even while changing the width-to-rim-diameter combination, keeps the speedometer reasonably accurate and avoids unintended side effects like altered gearing feel or reduced ground clearance from a smaller-than-stock overall tire diameter.
Your speedometer calculates speed based on wheel rotation speed multiplied by an assumed tire circumference set at the factory; if your actual tire's circumference differs from that assumption (from a tire size change), your speedometer will read slightly high or low compared to your true speed.
It's the tire's sidewall height expressed as a percentage of its section width — a lower aspect ratio (like 35) means a shorter, stiffer-looking sidewall relative to the tire's width, common on sportier setups, while a higher one (like 65) means a taller sidewall.
Because a significantly different overall diameter changes your effective gearing, fuel economy, speedometer accuracy, and ground clearance — most manufacturers recommend keeping any tire size change within about 3% of the original overall diameter.