What Does the Tyre Circumference & Speed Calculator Do?
A tyre-size code contains enough information to calculate a nominal sidewall height, overall diameter and circumference. This tool also converts wheel RPM into geometric road speed and shows revolutions per mile. It makes the dimensions behind a familiar size such as 235/40 R18 visible, which is useful for comparing gearing assumptions or checking a theoretical wheel-speed relationship.
How to Use This Calculator
- Read the tyre size code. Enter nominal width in millimetres, the aspect-ratio percentage and rim diameter in inches. For 235/40 R18, the values are 235, 40 and 18.
- Do not enter 40 mm as the sidewall height: the 40 is a percentage of width. The calculator derives sidewall height before adding both sidewalls to the rim diameter.
- Enter wheel RPM if you want a geometric road-speed output, or zero if that comparison is not needed. Wheel RPM is not engine RPM; gearing separates the two.
- Calculate and inspect diameter, circumference and revolutions per mile. Use measured rolling information for calibration work and the separate comparison tool when evaluating a change between two sizes.
The Maths & Assumptions Behind the Tool
Sidewall height = width × aspect percentage ÷ 100. Rim diameter in millimetres = inches × 25.4. Overall diameter adds two sidewalls to that rim diameter, and circumference is diameter × pi. Revolutions per mile = 1,609,344 ÷ circumference in millimetres. Speed in mph = wheel RPM × circumference × 60 ÷ 1,609,344. These equations describe nominal circular geometry and assume the supplied wheel RPM corresponds to rolling without slip.
Worked example
For 235/40 R18, sidewall height is 235 × 0.40 = 94 mm. Wheel diameter is 18 × 25.4 = 457.2 mm. Overall diameter is 457.2 + 2 × 94 = 645.2 mm; nominal circumference is 2,026.96 mm.
This gives approximately 794 revolutions per mile. At 800 wheel RPM, geometric road speed is 60.46 mph. Loaded rolling circumference can differ; use measured or manufacturer data for calibration work.
Why This Matters for UK Drivers
The first number is nominal section width in millimetres. The second is sidewall height as a percentage of that width, not an absolute millimetre measurement. The final number is rim diameter in inches. The calculator converts the rim into millimetres before adding the two sidewalls. Enter wheel RPM for the speed calculation; engine RPM must first be divided by the relevant gear and final-drive ratios.
Nominal circumference differs from effective loaded rolling circumference. Tyre model, pressure, load and wear can affect actual dimensions and distance travelled per revolution. The calculated speed therefore describes geometry under stated assumptions, not a calibrated speedometer reading. For approved replacement sizes or loading limits, consult the vehicle and tyre specifications. A mathematically similar diameter does not establish compatibility with the wheel, suspension, bodywork or braking system.
Interpreting the result in practice
The nominal tyre code is a useful starting point, but real rolling behaviour is not identical to a rigid unloaded circle. Load, inflation, tread depth and tyre construction can affect the effective distance travelled per revolution. Tyres with the same nominal code can also differ in detailed dimensions. For calibration or a close clearance decision, obtain manufacturer data or an appropriate measurement rather than relying on the code alone.
Revolutions per mile and circumference move in opposite directions. Increasing circumference by 2% reduces revolutions per mile by a factor of 1 divided by 1.02, approximately 1.96%. At unchanged wheel RPM, geometric speed increases by 2%. Keeping the reciprocal relationship clear helps explain why a larger tyre changes both gearing and indicated-speed assumptions.
This tool does not evaluate tyre load rating, speed rating, rim-width compatibility, body clearance or approval for a particular vehicle. Those properties are not encoded in the three dimensions entered. A mathematically similar circumference does not make two tyres interchangeable. Use the output to understand geometry, then verify any practical tyre choice separately.
The aspect ratio is relative to width, so the same percentage does not mean the same sidewall height on every tyre. A 40-profile tyre with 235 mm width has a nominal 94 mm sidewall; a 40-profile tyre with 255 mm width has a nominal 102 mm sidewall. With the same rim diameter, that wider tyre also has a larger nominal overall diameter.
This is why changing only the first number in a tyre code can affect gearing and circumference. To compare complete alternatives, enter all dimensions rather than assuming the shared rim size guarantees the same outside diameter. The separate size-comparison tool expresses the change as a percentage and shows the related speed assumption.
Keep units explicit when checking the formula by hand. Rim diameter begins in inches while sidewall dimensions are in millimetres. Converting the rim by 25.4 before adding the sidewalls is essential; adding the raw inch number would create a meaningless result.
Nominal size versus rolling circumference
The calculated circumference is geometric. A loaded tyre deforms and the effective rolling radius can differ from the unloaded dimensions, so real speedometer calibration and gearing calculations may not match perfectly.
Use this before a tyre-size comparison
If you are changing size, use the dedicated comparison calculator to show percentage diameter and speedometer differences directly.