Data sources
Where the numbers come from
All tire size calculations on TreadSize derive from a single source: the standard ETRTO (European Tyre and Rim Technical Organisation) dimensional formulas, which define how to compute overall diameter, circumference, and revolutions per mile from a tire size code. These formulas are the same ones used by tire engineers worldwide and are not proprietary.
Core formulas (metric)
sidewall_mm = section_width_mm × (aspect_ratio / 100)
rim_mm = rim_diameter_in × 25.4
OD_mm = rim_mm + 2 × sidewall_mm
circumference_mm = π × OD_mm
revs_per_mile = 1,609,344 / circumference_mmFlotation tires
For flotation sizes (e.g., 33x12.50R15), the overall diameter and section width are given directly in inches and converted:
sidewall_in = (OD_in - rim_in) / 2
OD_mm = OD_in × 25.4Speedometer calculation
The speedometer error formula is derived from the ratio of rolling circumferences:
actual_speed = indicated_speed × (OD_new / OD_stock)Plus sizing algorithm
Plus size candidates are generated by applying the standard rule (−10 aspect ratio points and optionally +10–20 mm width per rim inch step) and filtering for candidates within ±4% diameter of the base size.
No external data
TreadSize does not query any external tire database or vehicle fitment guide. All computations are deterministic algorithms run locally in your browser. We do not have, and do not claim to have, a verified database of vehicle-specific fitment data.
Accuracy limitations
The formulas produce nominal values — the values the size code implies. Actual manufactured tire dimensions vary by brand, model, rim width, and inflation pressure, typically within ±1–3% of the nominal. Always use measured data from the actual tire manufacturer when precision is critical.