The biggest change in tire technology is actually a return to the past. Early tires were solid rings of rubber that wrapped around wagon wheels. These tires were hard riding, but also impervious to punctures and very durable -- ideally suited to the rough terrain of the day. Today, tire manufacturers are trying to make the flat tire a distant memory.
Today's run-flat tires contain air and are much more complicated in construction than early rubber rings. But, they operate on the same principle: They are tough enough to run without air if necessary.
The above diagram shows the difference in construction between a conventional tire and a stiff-sidewall run-flat tire. In a stiff-sidewall run-flat tire, there is extra sidewall reinforcing rubber that prevents the sidewall of the tire from deflecting. In an inner-liner run-flat, the sidewall is no stiffer than a conventional tire, but a hard rubber or plastic ring inside the tire helps keep the tire's sidewall from deflecting.
There are two different styles of run-flat tires. The first uses very stiff sidewalls that can support the weight of the vehicle in the event of pressure loss. Several tire companies offer this type of run-flat, and they can typically be driven with no air pressure for about 50 miles (80.5 kilometers) at speeds up to 55 miles per hour (88.5 kilometers per hour). However, most cannot be repaired after being punctured. The sidewalls can't be very tall, so most are low-profile designs. Because of this, they are typically used on sports cars, though they're also available for regular passenger cars and minivans.
The second style is called the PAX system and was invented by Michelin. The PAX system isn't just a tire. It's a tire/wheel package that consists of four components: a tire, a wheel, an inner support ring, and a tire-inflation monitor. If the PAX system tire loses air pressure, it only drops about halfway down. At that point, the underside of the tread rests on an inner support ring that runs around the circumference of the wheel. According to Michelin, the vehicle can be driven for 125 miles (201.2 kilometers) at up to 55 miles per hour (88.5 kilometers per hour).
The PAX system also incorporates a special bead -- the connection between tire and wheel -- that helps lock the tire onto the wheel even if the tire loses air pressure, something traditional run-flat tires -- and regular tires -- don't usually have. Unlike most run-flat tires, the PAX-system tire can be repaired if the hole is in the tread area and less than 1/4-inch (6.4-millimeter) in diameter -- as is the case for regular tires.
Because the sidewalls of an un-inflated PAX tire don't support the weight of the vehicle in the event of pressure loss, the sidewalls can be taller than on run-flat tires. They also don't need to be as stiff, which means that the ride quality is better. This makes the PAX system better suited to SUVs, as well as regular passenger cars and minivans.
Go on to the next page to find out about selecting the right tire for the best fuel economy.