How this instrument works
The code stamped on a tire's sidewall, like 225/50R17, packs three measurements into a short string: 225 is the section width in millimeters (the tire's width at its widest point), 50 is the aspect ratio as a percentage (the sidewall's height relative to that width), and 17 is the rim diameter in inches. None of those three numbers alone tells you how tall the tire actually stands or how far it travels per rotation — you need to combine all three.
The sidewall height comes first: section width times aspect ratio, giving a height in millimeters (225mm x 50% = 112.5mm for a 225/50R17). That height is added twice — once above the rim, once below it, since the rim sits inside the tire — to the rim's own diameter, after converting millimeters to inches, to get the tire's overall diameter. From there, circumference is just diameter times pi, and revolutions per mile is 63,360 (the number of inches in a mile) divided by that circumference in inches.
This geometry matters whenever you change tire size on a vehicle: a taller overall diameter means the speedometer reads slow relative to true speed (since each rotation now covers more ground than the odometer assumes), while a shorter one makes it read fast. It also determines fitment — how much clearance a tire leaves in the wheel well — and matters for anyone comparing a stock tire size against a plus-sized aftermarket option before buying.
- Enter the section width in millimeters — the first number in the tire code (225 in 225/50R17).
- Enter the aspect ratio as a percentage — the second number (50 in 225/50R17).
- Enter the rim diameter in inches — the number after the R (17 in 225/50R17).
- Read sidewall height, overall diameter, and circumference.
- Read revolutions per mile — useful for comparing how a size change affects speedometer accuracy or gearing.
Worked example — a 225/50R17 passenger tire
A common 225/50R17 tire: sidewall height = 225 x 50 / 100 = 112.5mm, which is 112.5/25.4 = 4.429in. Overall diameter = 17 + 2x4.429 = 25.858in. Circumference = 25.858 x pi = 81.236in.
Revolutions per mile = 63,360 / 81.236 = 779.9 — meaning this tire turns over roughly 780 times to cover one mile of road, a figure that changes noticeably if you swap to a taller or shorter tire on the same wheel, which is exactly why gearheads run this calculation before buying a plus-sized replacement.
Questions
What do the numbers in a tire size code like 225/50R17 mean?
225 is the section width in millimeters (the tire's widest point, measured sidewall to sidewall), 50 is the aspect ratio as a percentage of that width (so the sidewall stands 50% as tall as the tire is wide), R indicates radial construction, and 17 is the diameter of the wheel rim the tire is built to fit, in inches. All three numbers are needed together to determine the tire's actual physical size.
Why does the sidewall height get added twice when finding overall diameter?
Because the rim sits in the middle of the tire, with sidewall rubber above it and below it. The wheel-and-tire assembly's total height is the rim's own diameter plus one sidewall height on top and another sidewall height on the bottom — hence the '2 x' in the formula (overall diameter = rim diameter + 2 x sidewall height).
How does changing tire size affect my speedometer?
A speedometer calculates speed from wheel rotation, calibrated to the vehicle's original tire size. A taller tire (larger overall diameter) covers more ground per revolution than the speedometer expects, so it will under-read your true speed; a shorter tire over-reads. Comparing revolutions-per-mile between your old and new tire size shows roughly how far off the reading will be.
Why do two tires with different size codes sometimes have almost the same diameter?
Because aspect ratio is a percentage of section width, not a fixed height — a wider tire with a lower aspect ratio can end up with nearly the same sidewall height, and therefore nearly the same overall diameter, as a narrower tire with a higher aspect ratio. Tire shops call this a 'plus-size' swap: going wider and lower-profile while keeping overall diameter close to stock.
Is aspect ratio always shown as a two- or three-digit number after the slash?
Yes, on the vast majority of passenger and light-truck tires sold today — a two- or three-digit percentage follows the section width, separated by a slash. A small number of older or specialty tires omit the aspect ratio entirely, in which case it's conventionally assumed to be a fixed default (commonly 82%), but that convention isn't part of this calculator's scope.