Tyre Size Calculator

How tyre size notation works

The standard metric notation — e.g. 205/55R16 — packs three numbers into one code: 205 is the tyre's section width in millimetres, 55 is the aspect ratio (sidewall height as a percentage of width — here, 55% of 205 mm = 112.8 mm), and 16 is the rim diameter in inches the tyre is built for. “R” simply means radial construction, which is standard on virtually every modern road tyre.

US-style pickup and 4x4 tyres often use flotation notation instead — e.g. 33x12.5R15 — where the first number is the overall diameter in inches (here, 838.2 mm), the second is the section width in inches, and the third is still the rim diameter in inches. The calculator parses both notations and converts flotation sizes to an equivalent aspect ratio automatically.

The diameter formula, step by step

Total tyre diameter = rim diameter + 2 × sidewall height, where sidewall height = width × (aspect ratio ÷ 100). For a 205/55R16: sidewall = 205 × 0.55 = 112.8 mm; diameter = (16 × 25.4) + (2 × 112.8) = 631.9 mm. The calculator shows every one of these steps for both your current and replacement size, so you can check the working, not just trust a black-box number.

From the diameter, circumference = diameter × π, and revolutions per kilometre = 1,000,000 ÷ circumference (mm). A 205/55R16 does about 504 revolutions per kilometre; a 225/45R17 with a larger 634.3 mm diameter does fewer — about 502 — because each rotation covers more ground.

The ±3% guideline — a workshop rule of thumb, not a law

A commonly used guideline among tyre fitters and online tyre calculators is to keep the overall diameter change to within about ±3% of the original size when swapping to a different tyre spec on the same rim. It's a practical rule of thumb — it is not an Australian Design Rule (ADR) or a legal standard — because a bigger change can throw off your speedometer/odometer accuracy, ABS and traction-control calibration, and clearance around the wheel arch or suspension components.

On the example above, a 205/55R16 to 225/45R17 swap is a +0.38% diameter change, which is inside that ±3% guideline. Being outside the guideline doesn't automatically mean the swap is unsafe or illegal — vehicle-specific fitment guides and a qualified tyre fitter are the final word — but it's a signal to double-check clearance and get a professional opinion.

Why your speedometer reads wrong after a tyre size change

Speedometers (and trip odometers) count wheel rotations, calibrated against the diameter of the tyre the vehicle shipped with. Fit a larger-diameter replacement and each rotation covers more ground, so your actual road speed is higher than the number on the dash — at an indicated 100 km/h with this example pair, you'd actually be doing about 100.4 km/h. A smaller-diameter replacement has the opposite effect: your real speed is lower than indicated.

This matters for speeding tickets (a mismatched speedo won't excuse a fine — traffic cameras and police radar measure your real speed) and for trip/fuel-economy readouts, which will also drift.

Ground clearance and other knock-on effects

A change in overall diameter changes ground clearance by half the diameter difference (the extra or reduced radius under the hub). For the example pair, that's about 1.2 mm more clearance with the larger 225/45R17. Bigger swaps can also affect gearing/acceleration feel, speedometer-linked systems (cruise control, some ABS/stability systems), and whether the tyre physically clears the wheel arch, guard liner or suspension strut at full lock and full compression — always check a fitment guide or ask a tyre shop before committing to a larger size.