Tire Basics

Tires are the only part of the vehicle that touches the road. Everything — acceleration, braking, steering, handling — happens through four contact patches each roughly the size of your hand. Understanding tires is understanding the foundation of vehicle dynamics.

A tire marked P225/65R17 breaks down like this. P means passenger vehicle. 225 is the section width in millimeters — the tire is 225mm wide from sidewall to sidewall. 65 is the aspect ratio — the sidewall height is 65 percent of the section width. R means radial construction. 17 is the wheel diameter in inches. Following the size you will find a load index number and a speed rating letter. The load index tells you the maximum weight each tire can support. The speed rating tells you the maximum sustained speed the tire is designed for. Never install tires with a lower load index or speed rating than the vehicle requires.

The correct tire pressure is on the placard on the driver door jamb — not on the tire sidewall. The number on the sidewall is the maximum pressure the tire can handle, not the recommended operating pressure. Underinflation causes the outer edges of the tread to carry most of the load, wearing the shoulders faster and generating excess heat. Overinflation puts all the load on the center of the tread, wearing the center faster and reducing the contact patch. Both reduce tire life and compromise handling.

Federal law requires TPMS on all vehicles sold after 2007. Direct TPMS uses a pressure sensor inside each tire that transmits wirelessly to the BCM. Indirect TPMS uses the ABS wheel speed sensors to detect a low tire by comparing rotational speeds — a low tire has a slightly smaller diameter and rotates faster. After any tire service, TPMS sensors must be relearned to the correct wheel positions. Always check whether the vehicle uses direct or indirect TPMS before performing tire service.