Reading a tyre size

A size such as 225/45 R17 has three geometric inputs. The first number is nominal section width in millimetres. The second is sidewall height as a percentage of width, so 45 means 225 × 0.45 = 101.25 mm. R17 means a 17-inch rim; 17 × 25.4 = 431.8 mm. Overall unloaded diameter is rim diameter plus two sidewalls. This calculator compares that estimate for an original and a replacement size; it does not certify that either tyre clears the vehicle.

A numerical comparison

Compare 225/45 R17 with 235/40 R18. The original sidewall is 101.25 mm and diameter is 431.8 + 202.5 = 634.3 mm. The replacement sidewall is 94 mm, while its 18-inch rim is 457.2 mm, so diameter is 645.2 mm. The change is 10.9 mm or about 1.72%. When the speedometer is calibrated for the original size and indicates 100 km/h, the larger rolling diameter travels about 101.7 km/h under the same wheel rotation assumption. Static clearance rises by half the diameter difference: roughly 5.45 mm.

Diameter, circumference and indicated speed

Tyres rotate once per circumference, and circumference is π × diameter. A larger circumference covers more road per wheel revolution, so an uncorrected speedometer and odometer read lower than actual travel. The calculator uses the diameter ratio, which is the same ratio as circumference because π cancels. It shows a theoretical speed change, not a calibration certificate. Tyre growth at speed, tread wear, inflation pressure, load, and the vehicle’s own speedometer tolerance all affect real readings.

Why clearance changes by half

Only the radius extends downward from the wheel centre. If diameter grows by 20 mm, the tyre’s static outer radius grows by 10 mm, so ground clearance rises approximately 10 mm. Clearance to struts, liners, brake components, and bodywork does not follow that simple rule because width, wheel offset, steering angle, suspension travel, and tyre shoulder shape matter. A narrower tyre can have a larger diameter; a wider tyre can have a smaller one. Compare the complete specification rather than assuming width predicts clearance.

Fitment has limits beyond geometry

A size may be close in diameter but still be unsuitable. Check the manufacturer’s approved rim-width range, load index, speed rating, wheel offset, bolt pattern, brake clearance, TPMS compatibility, and the vehicle placard or local regulations. A tyre rubbing only at full lock or with passengers may look fine while parked. Do not use this page to choose a lower load index, change staggered all-wheel-drive rolling circumferences without guidance, or override an insurer, inspection rule, or vehicle manufacturer requirement.

Nominal dimensions are not laboratory dimensions

Tyre width and diameter are standardised labels with tolerances, and different models can measure differently on the same rim. A 235 mm nominal tyre does not promise exactly 235 mm at every point. Tread depth also reduces as a tyre wears, while pressure and load change the loaded rolling radius. The calculation is best for screening whether a proposed change is roughly within a small percentage of the original. For a tight wheel arch or performance application, consult the tyre data sheet and measure the actual vehicle.

Mistakes that give convincing wrong answers

Do not enter 45 as 0.45 in the aspect field: this calculator expects a percentage and divides by 100. Do not leave the rim as 17 when the new wheel is 18 inches. Mixing millimetres with inches before conversion doubles the chance of error. Another mistake is interpreting the shown speed at an indicated 100 km/h as a legal correction for every speed. It is a proportional comparison under a simplified rolling model, not proof of speedometer compliance.

How this differs from the tyre size page

The existing Tire Size Calculator describes one tyre’s unloaded diameter from width, aspect ratio, and rim. This comparison page accepts two complete sizes and derives diameter difference, indicative speedometer effect, and approximate clearance change. It is not an EV charging calculator: both use vehicle inputs, but tyre geometry affects rolling distance while charging cost depends on battery energy, tariff, efficiency, and state of charge. Keeping those calculations separate makes the assumptions auditable.

Use the result as a screening step

Enter the placard size exactly, then enter the candidate size and compare the percentage before buying. Recheck the wheel rim width and offset, load and speed ratings, and clearance at steering lock and suspension compression. If the difference is material, seek vehicle-specific fitment advice and consider recalibration requirements. The browser does all arithmetic locally; it does not send a tyre choice anywhere, but it also cannot observe a real wheel arch, brake package, drivetrain, or the legal rules that apply where the vehicle is used.

One last practical check

For vehicles with driven axles that must rotate at closely matched speeds, compare rolling circumference across all fitted tyres, including remaining tread depth, rather than changing only one corner. Replacement tyres should normally be a matched set on an axle and follow the vehicle maker’s instructions for rotations and repairs. Check the spare tyre separately; a temporary spare has its own restrictions. A small calculated percentage is useful for shortlist screening, but the final selection should be the exact tyre model, wheel, and vehicle combination approved for the load and operating conditions.

Tire Size Comparison