Nominal Section WidthMetric Cross-Section Dimension (mm)
The linear width in millimeters between the exterior sidewalls of an inflated tire at its widest point, excluding protective ribs or lettering. Passenger tire widths span 145 mm to 335 mm. Under ETRTO standards, mounted section width increases or decreases by approximately 5 mm for every 0.5-inch deviation from the manufacturer measuring rim width.
Default: 215 mm
Aspect Ratio ProfileSidewall Height Ratio Percentage (%)
The dimensional relationship expressing radial sidewall height as a percentage of nominal section width. A 45 aspect ratio denotes a sidewall height measuring 45% of width. Lower profiles (30% to 45%) enhance cornering precision and lateral stiffness by curbing sidewall deflection, while taller profiles (55% to 75%) absorb road shocks and shield wheel rims from pothole impacts.
Default: 55%
Wheel Rim DiameterHub Mounting Seat Flange (inches)
The nominal diameter in inches of the wheel rim cylinder supporting the tire bead. Standard passenger and SUV wheels range from 14 to 22 inches. When plus-sizing to larger rim diameters, aspect ratio must decrease proportionally to preserve original overall rolling diameter within the safe +/- 1.5% threshold.
Default: 17 inches
Overall Rolling DiameterTotal Unladen Diameter (inches / mm)
The outer diameter from top to bottom tread surface: (Section Width * Aspect Ratio * 2 / 25.4) + Rim Diameter. The Tire and Rim Association (TRA) specifies that replacement tire outer diameter should remain within +/- 1.5% (maximum 3.0% on 2WD) of OEM dimensions to preserve transmission shift points, ABS calibration, and suspension clearance.
Default: 26.3 inches (668 mm)
Speedometer Indication VarianceVelocity Discrepancy Calibration (mph / km/h)
The discrepancy between instrument cluster speedometer readout and true radar ground velocity. Because speed sensors tally axle rotations, larger tires cover greater distance per revolution, causing the speedometer to under-report actual vehicle speed. UNECE Regulation 39 mandates that production speedometers must never under-read true ground speed.
Default: 0.0 mph / km/h variance
Wheel Offset (ET) and BackspacingWheel Centerline Offset (ET mm / inches)
The distance in millimeters from the wheel longitudinal centerline to the hub mounting face. High positive offset (ET+45) draws the wheel inward toward the suspension strut. Lower positive or negative offset (ET+20 or ET-10) pushes the wheel outward toward the fender lip ('poke'). Deviating by more than 5 to 10 mm alters steering scrub radius and increases wheel bearing load.
Default: ET +45 mm
Inner Suspension & Strut ClearanceChassis Clearance Margin (mm)
The physical clearance buffer between the tire inner sidewall and the suspension strut tower, spring perch, or steering knuckle. A wider tire or wheel with increased positive offset reduces this gap. Chassis engineering standards mandate maintaining at least 8 to 10 mm of dynamic clearance under full suspension travel and steering lock.
Default: 15.0 mm nominal clearance
