Automakers would love to shed the weight, cost and bulk of full-sized or mini spare tires, and it's starting to look like they've found a preferred alternative: tire repair kits.
According to Edmunds.com, 70 car models sold in the United States in the 2012 model year had tire repair kits, up from 23 in 2009.
"It's getting harder and harder for automakers to meet fuel economy regulations," says Ronald Montoya, an editor at Edmunds.com. "They are doing everything they can to take weight out of the car, and the spare tire is one of the easier targets."
There's just one problem: It's unclear whether motorists are willing to give up the spare.
Ford Motor Co. and General Motors have introduced tire repair kits, only to restore the spare on some models to mollify car buyers.
A tire repair kit includes a compressor that is powered by an electrical cord connected to the car's 12-volt outlet.
The compressor pumps air and liquid latex sealant into the tire, which is reinflated in five to 10 minutes. Such repair kits typically cost $40 to $100 in the aftermarket.
They have drawbacks:
• The sealant can damage sensors inside some tires.
• The motorist has to buy a new kit to replace the used sealant.
• Repair kits provide only a temporary fix. The motorist must take the tire to a shop for a permanent repair as soon as possible.
Despite their flaws, tire repair kits are eclipsing run-flat tires as a leading candidate to make spare tires obsolete.
Run-flats were introduced in the 1990s, and once were considered a leading candidate to eliminate the spare. The most popular type is the SSR, or the self-supporting run-flat tire. After a puncture, rubber inserts inside the tire reinforce the sidewall, maintaining the tire's shape and allowing the motorist to continue driving at a reduced speed.
Typically, the motorist can drive on it as far as 50 miles or so before taking it to a repair shop.
BMW installs run-flats on a variety of models, but mass-market automakers seem reluctant to follow suit. Honda Motor Co., for example, no longer uses run-flats on the Odyssey minivan or the Acura RL.
Part of the problem is cost. Replacement run-flats can cost more than $200 apiece, compared with $100 or so for a good conventional tire.
Run-flats often have higher rolling resistance than ordinary tires, and they sometimes wear out quickly.
According to Edmunds.com, only 29 U.S. models sold in the 2012 model year had run-flat tires as original equipment.
While that's more than in 2007, when only 16 models had them, Montoya of Edmunds.com says run-flats have been relegated to a niche market.
"You only see run-flats on luxury cars and sports cars," Montoya said. "We expect to see more tire repair kits" in the future.
Another alternative is self-sealing tires, which have a sticky interior coating that can repair punctures up to 0.2 inches in diameter. In 2008, Volkswagen equipped the Passat with Continental Tire's ContiSeal self-sealing tire.
Self-sealing tires are relatively new, and automakers are still evaluating them.
Continental Tire sells 1.5 million ContiSeal tires annually, but the company isn't betting on any one particular tire technology, says r&d director Alessandra Ferraris.
"We offer a complete portfolio of products," Ferraris said, "and we will continue offering it in the future."
For the moment, the unglamorous mini spare tire remains the industry's chief alternative to full-sized spares. According to Edmunds.com, 54 percent of all 2012 models sold in the United States had mini spares.
But mini spares have poor handling characteristics, and they shouldn't be driven more than 50 miles. They also add weight and bulk, which is why automakers are tinkering with tire repair kits.
So far, they have had mixed results. In the 2011 model year, GM replaced the spare in the Chevrolet Cruze with a repair kit, only to switch back the following year to a mini spare. And in the 2008 model year, Ford introduced a repair kit for the Ford Focus, only to replace it with a mini spare tire later that model year.
"Consumers did not accept the technology," said Dave Rohweder, Ford's global chief tire engineer. "There is still a chunk of customers who expect to see a full-sized tire in the trunk."