How we calculate

A metric size such as 225/65R17 gives section width in millimetres, aspect ratio as a percentage of that width, and wheel diameter in inches. The R identifies radial construction. Load index and speed symbol describe service ratings; they are not inputs to the diameter calculation.

• Sidewall (mm) = width (mm) × aspect ratio ÷ 100.
• Sidewall (in) = sidewall (mm) ÷ 25.4.
• Overall diameter (in) = wheel diameter + 2 × sidewall (in).
• Section width (in) = width (mm) ÷ 25.4.

Example, 225/65R17: sidewall = 225 × 65 ÷ 100 = 146.25 mm, or 5.758 in; diameter = 17 + 2 × 5.758 = 28.516 in, shown as 28.52 in (724 mm); section width = 225 ÷ 25.4 = 8.86 in.

Flotation sizes state nominal overall diameter, section width and wheel diameter directly. For 33x12.50R15, diameter is 33 in, width is 12.50 in, and sidewall = (33 − 15) ÷ 2 = 9 in.

• Circumference (in) = π × overall diameter (in).
• Revolutions per mile = 63,360 ÷ circumference (in), because a mile has 63,360 inches.
• New-to-current ratio = new unrounded diameter ÷ current unrounded diameter.
• Actual speed = indicated speed × new-to-current ratio.
• Actual miles when the odometer counts 1,000 = 1,000 × new-to-current ratio.

Examples: for 225/65R17 to 235/60R18, circumference is π × 28.516 = 89.59 in and 63,360 ÷ 89.59 = 707 revolutions per mile. The new diameter is 29.102 in, so the ratio is 29.102 ÷ 28.516 = 1.0205. At an indicated 60 mph, estimated actual speed is 60 × 1.0205 = 61.2 mph; 1,000 odometer miles correspond to about 1,021 miles driven. The two tires calculate to 707 and 693 revolutions per mile. For flotation size 33x12.50R15, circumference is π × 33 = 103.67 in and 63,360 ÷ 103.67 = 611 revolutions per mile; its 33.00 in diameter and 9.00 in sidewall are nominal.

A larger replacement tire makes the vehicle travel farther per wheel revolution, so the speedometer and odometer tend to read low relative to actual travel. A smaller tire tends to make them read high. These are geometric estimates; vehicle calibration can also affect readings.

• Diameter difference = new diameter − current diameter.
• Diameter change (%) = diameter difference ÷ current diameter × 100.
• Ride height change = diameter difference ÷ 2.
• With the same wheel offset, each tread edge moves by half the section-width difference.

Example: 29.102 − 28.516 = 0.586 in, or +2.1%; the center of the axle rises by 0.586 ÷ 2 = +0.293 in, shown as +0.29 in. Width changes from 225 mm to 235 mm, a 10 ÷ 25.4 = 0.394 in increase, so each tread edge moves about 0.197 in outward, shown as +0.20 in, assuming the same offset.

The verdict uses the absolute, unrounded diameter percentage against the current tire: up to and including 3.0% is “Within 3%: a common safe change”; over 3.0% through 5.0% is “3–5%: check clearance and speedometer”; over 5.0% is “Over 5%: not recommended without changes.”

Three percent is a commonly used screening rule of thumb for keeping overall diameter and gearing close during a tire change; it is not a regulatory fitment limit or a guarantee of safety. Tire and wheel references explain size conventions and the need to select suitable tires and rims, while vehicle clearance, load capacity, handling and calibration depend on the specific vehicle and tire. Always verify those details for the actual combination.

The size comparison calculator is a screening aid. A similar diameter does not confirm fitment. Check load index, approved wheel width and clearance.

For a valid current size, the calculator checks the same wheel diameter and the next two wheel diameters (+1 and +2). It keeps sizes from the shared COMMON_SIZES list whose diameter change rounds to within 3.0% at the displayed one-decimal precision, sorts by the absolute unrounded difference, and shows up to four per wheel diameter. The current size is excluded.

The shared list used by the chart and plus-size suggestions is grouped by wheel diameter (in):

• 15 in: 195/65R15, 205/65R15, 215/70R15, 235/75R15, 31x10.50R15, 33x12.50R15
• 16 in: 205/55R16, 215/60R16, 225/70R16, 245/75R16, 265/75R16, 285/75R16
• 17 in: 215/55R17, 225/45R17, 225/65R17, 235/65R17, 245/65R17, 265/70R17, 285/70R17, 35x12.50R17
• 18 in: 225/40R18, 225/45R18, 235/60R18, 245/60R18, 255/55R18, 265/65R18, 275/65R18, 275/70R18
• 19 in: 235/55R19, 245/45R19, 255/50R19
• 20 in: 245/45R20, 255/55R20, 275/55R20, 275/60R20, 275/65R20, 285/55R20
• 22 in: 275/50R22, 285/45R22

This list is a curated set of common examples, not a complete catalog or a recommendation that a listed size fits a particular vehicle. Plus sizing can change load capacity, ride, handling and cost; verify tire and wheel specifications with the vehicle and tire manufacturers.

Wheel width is measured between bead seats. The calculator estimates the wheel edges from the nominal width and offset, using millimetres for offset and 25.4 mm per inch.

• Backspacing (in) = (wheel width (in) + 1) ÷ 2 + offset (mm) ÷ 25.4.
• Inner edge change (mm) = (new width × 25.4 ÷ 2 + new offset) − (old width × 25.4 ÷ 2 + old offset).
• Outer edge change (mm) = (new width × 25.4 ÷ 2 − new offset) − (old width × 25.4 ÷ 2 − old offset).

Example, 8 in +35 mm to 9 in +20 mm: backspacing changes from (8 + 1) ÷ 2 + 35 ÷ 25.4 = 5.88 in to (9 + 1) ÷ 2 + 20 ÷ 25.4 = 5.79 in. Inner change = (9 × 25.4 ÷ 2 + 20) − (8 × 25.4 ÷ 2 + 35) = −2.3 mm, meaning the inner edge is 2.3 mm farther from the suspension. Outer change = (9 × 25.4 ÷ 2 − 20) − (8 × 25.4 ÷ 2 − 35) = +27.7 mm, farther outward.

All differences, ratios, percentages, speeds and thresholds are calculated from unrounded dimensions. Rounding is applied only for display: inches to 2 decimal places; millimetres to whole numbers; revolutions per mile to whole numbers; percentages to 1 decimal place with a plus or minus sign (zero is 0.0%); and speeds to 1 decimal place. Wheel edge changes are shown to 1 decimal millimetre and backspacing to 2 decimal inches.

For example, the 225/65R17 to 235/60R18 diameter difference is calculated as 29.102 − 28.516 = 0.586 in and then displayed as +0.59 in. Its percentage is calculated from the unrounded values, 0.586 ÷ 28.516 × 100, then displayed as +2.1%.

• Dimensions are nominal sidewall-based estimates for unloaded tires. Actual measured dimensions vary by tire model, rim width, pressure, wear, load and operating conditions.
• Metric calculations use the printed width and aspect ratio; flotation calculations use the printed nominal diameter and width.
• Clearance estimates use diameter and section width only; they do not model tread shape, steering angle, suspension travel, wheel offset changes or vehicle geometry.
• The site does not confirm fitment for any vehicle. It does not check vehicle-specific load requirements, approved rim ranges, brakes, body clearance or legal requirements.

Use the vehicle placard and manual, tire and wheel manufacturer specifications, and a qualified professional to verify a proposed setup.