Data sources

Dimensions from the size marking

The dimensional calculators use the width, aspect ratio, and rim diameter in a tire-size marking. These are nominal geometric calculations, not measurements of an actual tire or a vehicle fitment database. They do not imply endorsement by a standards organization.

Metric geometry

sidewall_mm = width_mm × aspect_ratio / 100
rim_mm = rim_in × 25.4
diameter_mm = rim_mm + 2 × sidewall_mm
circumference_mm = π × diameter_mm
nominal_revs_per_mile = 1,609,344 / circumference_mm

For 215/55R17, the nominal diameter is 668.3 mm. Manufacturer-published dimensions and rolling revolutions may differ. The Tire Rack explanation of tire dimensions provides additional context for this calculation.

Flotation sizes

A marking such as 33x12.50R15 supplies nominal overall diameter, width, and rim diameter in inches.

sidewall_in = (diameter_in - rim_in) / 2
millimeters = inches × 25.4

Speed and distance estimates

ratio = new_diameter / original_diameter
estimated_actual_speed = indicated_speed × ratio
estimated_actual_distance = indicated_distance × ratio
odometer_reading_error_% = (1 / ratio - 1) × 100

These estimates assume the original speed and distance calibration was accurate and remains unchanged. They do not account for actual rolling circumference, loading, inflation, wear, or existing vehicle calibration error.

Plus-size candidates

The generator tries +1 and +2 rim increments, widths equal to the base or 10–20 mm wider, and lower aspect ratios. It retains a limited set within ±3% nominal diameter difference. This is a tool-specific filter, not a safety standard, an exhaustive list, or a check of product availability.

Vehicle and tire information

The original size is supplied by the user. TreadSize does not perform a VIN or trim lookup. Use the vehicle placard, owner's manual, and manufacturer specifications to establish the correct baseline. NHTSA's tire guidance explains the placard and recommended tire size.

Load index, speed rating, approved rim width, wheel offset, tire-model dimensions, and physical clearance are not established by the dimensional calculation. The complaints dataset and recall dataset are separate reference data; their methods and sources are described on those pages.

Using the result

Use nominal dimensions to understand a change and prepare questions. Use the actual tire manufacturer's specifications and a qualified installer to verify a proposed installation.