Slowly but surely, spare tires are being phased out as original equipment on new vehicles, as auto makers turn to run-flat tires or inflation kits to save space and reduce weight.

Run-flats, which have been on the market for years, are on average nearly twice the price of traditional tires. They also affect vehicle performance, making it necessary for auto makers to work closely with tire suppliers during development of a vehicle, says Tom Chubb, vice president of marketing-Michelin Automotive Industry Div.

“The tuning of the (vehicle's) suspension has to be addressed,“ he tells Ward's. “The tire is one of the most tunable elements in the chassis system. It's right (up) there with shocks and bushings.”

As for the high price of run-flats, Chubb says they will come down, but never be as affordable as traditional tires due to the extra material used in their production.

Yet a handful of auto makers today offer run-flats as standard equipment in lieu of traditional spares.

BMW AG is one such auto maker, offering run-flats standard on a number of vehicles, including its bread-and-butter 3-Series, which ditched its spare and was shod with run flats in '04, a spokesman says.

Poor handling is a problem that has historically plagued run flats due to their increased rigidity, especially in the sidewalls, which allows them to retain their structural integrity following a blow out.

When first offered, some performance issues creeped up on BMWs equipped with run-flats, the spokesman says, but the problems have since been corrected.

Now, the greatest issue is educating consumers on the advantages of run-flats, he says, as some buyers remain wary about giving up their spares.

“There are customers that feel comfortable having a spare on their car and this is new technology and we have to prove that it does work,” the spokesman says.

As the name suggest, run-flats are designed to bear the weight of the vehicle even when the pressure within the tire is greatly reduced, which sometimes makes it difficult for drivers to even know they have a flat, the BMW spokesman says.

BMW addressed that problem inherent to run-flat tires by installing a warning light, he says.

“It's difficult to tell when you have a flat because the handling doesn't go away and it drives the same way,” he says. “Only if you were really in tune with your car could you tell (you had a flat).

The flat-tire indicator light, located on the instrument panel, is difficult to miss, the spokesman says, referring to it as “quite blatant.”

Despite some of the problems associated with run-flats, their many advantages fueled speculation that their industry penetration would occur at a rapid case.

Not so, says Chubb.

“Over the last 5 years the demand for run-flats hasn't developed as much as we thought it would have,” he says. “And the reason for that is tire-pressure monitoring systems, which are federally mandated now.”

In many cases, tire-pressure monitoring systems can help a driver safely reach a repair shop without the need of a spare, run-flats or a tire-inflation device, Chubb says.

“Over 80% of tire-pressure losses are slow, and as a result of that the consumer has a fairly significant amount of time to investigate the problem and get it fixed without being stranded,” he says. “That changed the perspective on run-flats.”

A cheaper alternative to run-flats are tire-inflation kits, usually composed of an air compressor and a canister of liquid tire filler. When a flat occurs, a motorist only needs to hook up the compressor, which automatically sprays the solution into the tire.

According to Continental AG, which produces such a kit dubbed “ContiComfortKit,” a repaired tire can be used at a maximum speed of up to 50 mph for up to 120 mph.

Continental and other suppliers have been selling such kits to OEMs for use as standard equipment for some time now with limited success.

Ford Motor Co. is an early adopter of the kits, currently offering them as standard equipment on the Ford GT super car, Volvo V50, S50, XC90 and the new '08 Ford Focus. BMW also offers the kits on a handful of vehicles.

“We're seeing a trend that people don't like to change their spare, they want to keep going,” says Kim Steele, chassis supervisor, tire engineering department, Ford.

“Run-flats will keep you going, but this (inflation kit) will get you back on the road in under 10 minutes, and that's what people want,” she tells Ward's.