Tire technology appears t be returning to its roots. Early tires were solid rings of rubber that wrapped around wagon wheels (like a child's tricycle wheel, today). While those tires were hard riding, they were durable and impervious to punctures durable. Late in the 1800s as automobiles were taking root, inflatable tires made for a much smoother ride on the rough roads of the era.
Today's tire manufacturers are trying to make the flat tire a distant memory. The "Run flat" tires contain air but are tough enough to run without air if necessary, using a stiff-sidewall. Some have extra sidewall reinforcing rubber and some have an inner-liner with a hard rubber or plastic ring inside the tire helps keep the tire's sidewall from deflecting.
The stiff sidewall run flats can support the weight of the vehicle and can typically be driven with no air pressure for about 50 miles at speeds up to 55 mph. However, most cannot be repaired after being punctured. These stiff sidewalls can't be very tall, so most are low-profile designs, and typically used on sports cars like the Dodge Viper, but are also available for regular passenger cars and even minivans.
The second style is a new design invented by Michelin dubbed the PAX system. It's a tire/wheel package that consists of four components: a tire, a wheel, an inner support ring, and a tire-inflation monitor. When the tire loses pressure, the tread rests on an inner support ring, and can be driven for 125 miles at up to 55 mph. It is currently offered in North America on Rolls Royces and on the 2005 Honda Odyssey minivan, with more vehicles expected in the future.
The PAX system has a special bead, connecting the tire and wheel, that works even when the tire loses air pressure. Unlike other run-flat tires (and just like regular tires), the PAX-system tire can be repaired if the hole is in the tread area and less than 1/4-inch in diameter. Because the sidewalls of an un-inflated PAX tire don't need to support the weight of the vehicle in the event of pressure loss, the sidewalls don't need to be as stiff and can be taller than on run-flat tires.
Because there's often little change in driving feel, drivers need to know the tire has lost pressure, so run-flat tires must be used with a tire-pressure monitoring system, which illuminates a light on the instrument panel to inform the driver of pressure loss.
Some manufacturers are turning to a less-costly but space saving alternative to spare tires: temporary mobility kits (TMK) which use an aerosol can with compressed air and liquid rubber. You connect the can to the valve stem of the tire, and the air and rubber are injected to seal the puncture and refill the tire. You can drive to the nearest gas station and get the tire repaired. The TMKs only work with a small puncture on tires that are still be roadworthy. Often flat tires are destroyed by drivers travelling too far while they are under-inflated, making the TMK useless. Often, though, the TMK can make a good tire with only a small hole irreparable, and the liquid rubber in the TMKs can also damage expensive wheels requiring complete replacement.