Basic Terms and Definitions
Learning about tyres begins with some terms and definitions that are commonly used in all aspects of the tyre industry. Knowing and understanding these terms is the foundation for learning about tyre sizing, sidewall nomenclature and all factors that affect tyre performance.
Passenger Car Tyre Sizing
Four sizing systems exist for passenger tyres today: European Metric, P-Metric, Millimetric and Alpha-Numeric. Each of these systems evolved from the first tyre sizing system – the Numeric Sizing system – which is now obsolete. It was developed when all tyres has the same aspect ratio and it provided only the nominal cross-section width of the tyre and the rim diameter in inches. The following are examples that identify the four different sizing systems.
- Eurpean Metric
- Used world wide. Aspect ratio appears in the size designation.
- Used mainly in North America. It evolved in the late 1970s in an attempt to standardise tyre sizing world wide and is based on the metric system.
- This system is used almost exclusively for TRX/TDX tyres.
- In the late 60s, this load based system evolved. The first letter of the designation indentifies the load/size relationsip of the tyre. The letter can range from “A” to “N”, with “A” being the lowest. The lower letter, the smaller the size and load-carrying capactiy at a given air pressure.
Load Index and Speed Rating
The load index is an assigned number ranging from 0 to 279 that corresponds with the maximum load carrying capacity of a tyre. Most passenger car tyres’ load indexes range from 62 to 126.
The speed index is an assigned letter ranging from J to Z that corresponds to the reference maximum speed at the associated load index. Refer passenger speed rating section below.
These two elements put together are called the Service Description and are mutually dependent.
The table below gives the load index and the speed symbol with their corresponding values.
Plus 1 and Plus 2 Sizing
As the chart illustrates, this concept achieves increased handling capabilities by mounting tyres with wider section widths and lower aspect ratios to rims 1, 2 and sometimes 3 inches greater in diameter.
Plus 1 and Plus 2 fitments must also retain nearly the same overall tyre diameter so that gear ratios and speedometer remain accurate. Equally important to proper Plus 1 and Plus 2 sizing is maintaining a load-carrying capacity that is equal to or greater than required on the placard.
A closer look at the concept shows how it achieves greater performance benefits. Wider section widths and lower profiles give tyres a wider footprint that increases the vehicle’s steering response and overall cornering force.
The wider wheel widths that fit Plus 1 and Plus 2 tyres strengthen their stability, also resulting in greater cornering force. It is important to select Plus 1 and Plus 2 wheels that meet recommended rim width range specifications. A wheel that is outside the recommended width range can curve the tyre’s tread surface. Properly mounting the tyre may also prove difficult.
Plus 1 Rule of Thumb*:
- Increase section width by 10 mm.**
- Decrease aspect ratio by 10 points.
- Increase rim diameter by 1 inch.
Plus 2 Rule of Thumb*:
- Increase section by 20 mm.**
- Decrease aspect ratio by 20 points.
- Increase rim diameter by 2 inches.
*These guidelines assume a base 70 series tyre.
**Check all guards and suspension components for proper clearance.