Methodology
Every number on this site is calculated from the tire size itself with standard geometry formulas. None of it is copied from manufacturer tables, and none of it is a load index, speed rating or other manufacturer-published rating.
Formulas
- Sidewall height = width × aspect ÷ 100
- Overall diameter (mm) = rim × 25.4 + 2 × sidewall
- Circumference = π × diameter
- Revolutions per mile = 63,360 ÷ circumference in inches
- Speedometer error % = (new diameter ÷ old diameter − 1) × 100
- True speed = indicated speed × new diameter ÷ old diameter (speedometer calibrated for the old size)
- Ride-height change = half the overall-diameter difference
Example, 265/70R17: sidewall = 265 × 70 ÷ 100 = 185.5 mm; diameter = 17 × 25.4 + 2 × 185.5 = 802.8 mm (31.61 in); circumference = π × 31.61 = 99.29 in; revolutions per mile = 63,360 ÷ 99.29 ≈ 638.
Flotation sizes
A flotation size such as 33x12.50R17 lists overall diameter (33 in), section width (12.50 in) and rim diameter (17 in). It is converted to metric equivalents: width = 12.50 × 25.4 = 317.5 mm; sidewall = (33 − 17) ÷ 2 × 25.4 = 203.2 mm; aspect = 203.2 ÷ 317.5 × 100 = 64. Page addresses keep the flotation form, lowercased with dots as hyphens (for example /tires/33x12-50r17/). Metric sizes show an approximate flotation equivalent (diameter × width, in inches).
Which sizes and comparisons are listed
The size list is a static compilation of common passenger, SUV and light-truck sizes as offered in major retailers' size menus. A size is a fact; every figure attached to it is computed here.
Comparison pages exist only for plausible swaps: the same rim diameter or a plus-one / plus-two fitment (up to 2 inches larger), with overall diameter within 5%. Each size links to its closest matches by diameter, plus the standard one-step width and aspect changes and same-rim metric/flotation swaps, capped at 5,000 comparisons in total. Each size page also lists every size within 3% of its diameter.
The "is this a safe swap?" check looks only at the diameter difference: under 1%, within the commonly used ±3% guideline, or outside it. It does not assess load index, speed rating, wheel width, offset, clearance, or vehicle calibration.
Rounding
Calculations use full floating-point precision. Displayed values are rounded: inches to 2 decimals, millimeters to 1 decimal, revolutions per mile to whole numbers, and percentages to 2 decimals. Actual tires vary by brand, tread depth, inflation and load.
Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.