Tires & Wheels

The ISO Metric Tire Code (standardized under ISO 4000-1 and ETRTO conventions, e.g. P225/45R17 91V) is the universal automotive tire sizing nomenclature. Decoding section width in millimeters, aspect ratio sidewall height percentage, internal radial carcass construction, wheel rim diameter in inches, load index, and maximum speed symbol.

The Tire Load Index (standardized under ISO 4000-1) is a numerical code assigned to a passenger or commercial tire indicating the maximum physical weight (in kilograms and pounds) the tire can carry when inflated to its maximum operating cold tire pressure. Spanning Load Index 70 (335 kg / 739 lb) to Load Index 120 (1400 kg / 3086 lb) per tire.

Tire Speed Ratings (standardized under ISO 4000-1 and ECE Regulation 30) are alphabetical index symbols indicating the maximum sustained velocity a tire can safely maintain under its rated load capacity. Ranging from Q (160 km/h / 99 mph) for dedicated winter tires to V (240 km/h), W (270 km/h), Y (300 km/h), and (Y) (>300 km/h / >186 mph) for hypercars.

Tire Pressure Monitoring Systems (TPMS, mandated by the US TREAD Act and EU UNECE Regulation 64) utilize Direct TPMS wheel-mounted valve sensors communicating via RF radio frequencies (315 MHz in North America, 433.92 MHz in Europe and Global). Utilizing the standard Schrader pneumatic valve with unified 8V1-32 UNEF cap threads.

Wheel Bolt Patterns (defined by the number of wheel lugs and their Pitch Circle Diameter / PCD in millimeters or inches, e.g. 5×114.3 mm or 5×4.5") establish the mechanical bolt hole circle on the wheel hub. Standardizing automotive fitments across Japanese (5×114.3), German VAG/Mercedes (5×112), BMW (5×120 / 5×112), Ford/PSA (4×108), and pickup trucks (6×139.7 / 6×5.5").

The Wheel Center Bore (measured in millimeters to 0.1 mm precision) is the machined center opening on the back of a wheel designed to fit snugly over the vehicle's raised wheel hub snout. Ensuring the wheel is mounted 'hub-centrically' (bearing vehicle weight on the center hub collar rather than the lug bolts), preventing high-speed vibrations.

Wheel Offset (denoted as ET from the German 'Einpresstiefe', measured in millimeters) is the distance from the wheel's true centerline to the mounting hub face. Dictating whether wheels tuck inside fenders (Positive ET), sit centered (Zero ET), or poke outwards (Negative ET), offset directly controls suspension scrub radius, steering stability, and brake caliper clearance.