They hide in the dark corners of our trunks. We don’t give them a second thought until we need them. And when we do, they come out to save the day. A spare tire is an unsung hero, able to get us back on the road in a matter of minutes. But what if you reached into the trunk and found no spare tire in your new car? What if, in its place, you found a shoebox-size device with a power plug?

Decades ago, nearly every car came with a full-size spare tire. But fuel economy requirements, trunk space considerations and safety concerns have prompted automakers to shift toward smaller temporary spares. In some vehicles, there’s no spare at all.

Although carmakers are working to come up with new spare tire options for their cars, the reality is that most people just reach for the phone when they have a flat.

“Our research says that only about half the people who have spare tires actually use them when they have a flat,” says Dave Cowger, engineering group manager for General Motors tire engineering. “Because of the convenience of roadside assistance, many of them make that call even if they have a spare.”

Whether you ultimately choose to change your flat tire or not, it’s good to familiarize yourself with the different types of spares as part of new-car shopping research. At the very least, it’s worth asking about the spare system when you’re finalizing a purchase. That way, you won’t be caught off-guard when you go looking for a spare that might not be there.

Full-Size Spares

A full-size spare is the same size as the other tires on the vehicle. Full-size spares come in matching and non-matching varieties. A matching spare is identical to the other wheels and tires on the vehicle. A non-matching full-size spare typically has a lighter-weight construction and a shallower tread depth that reduces vehicle weight to improve fuel economy and make the spare easier to install, according to Tire

There has been a 49 percent decrease (accounting for about 64 models) in vehicles that come equipped with full-size spares since 2007. Except for trucks or larger SUVs, full-size spares have disappeared from the majority of passenger cars.

While they are the biggest and heaviest of the spares, full-size tires offer virtually no performance loss. If you have a matching spare, you have the flexibility to get the tire fixed at your convenience, rather than immediately, which you would have to do if you had a temporary spare. Full-size spares must be incorporated into the vehicle’s rotation pattern to ensure a long tread life and balanced handling characteristics.

Temporary Spares

Automakers have shifted to temporary spare tires or “donuts” because they offer the best balance between size and usability. Temporary spares are the most common choice for automakers and can be found on 52 percent of 2014 models. That’s a 7 percent increase from 2007, according to specifications tracked by the Edmunds data team. Temporary spares are smaller than the vehicle’s other tires, take up less trunk space and are light enough for most people to handle when they’re changing a flat.

“Temporary spares are developed to approximate your vehicle’s optimum handling,” says Kurt Berger, manager of consumer products engineering for Bridgestone Tire, which provides tires for many automakers. “The carmaker and tire maker go out of their way to develop that temporary spare to minimize the handling difference.”

But since they are not quite equal to your regular tires, they are only meant to be driven a short distance. Most temporary spares also are limited to a speed of 55 mph.

Run-Flat Tires

Vehicles equipped with run-flat tires have no spare at all. Run-flats typically have reinforced sidewalls that allow them to operate with little to no tire pressure.

Once a run-flat tire has been punctured and lost its air pressure, the vehicle’s tire-pressure monitoring system (TPMS) will notify you that the tires are below spec and the vehicle is in “run-flat mode.” You will have to keep your speed under 50 mph, but you can still continue to drive (up to 50 miles) until you are able to find a tire shop or service station.

The advantage of run-flats is that “you don’t have to stop your vehicle in an inopportune time or dangerous location, like the side of the freeway,” Berger says.

Most run-flat tires can be repaired unless they are specifically marked otherwise. If a driver needs a replacement, run-flats are often more expensive than standard tires. Additionally, not every tire shop carries run-flat tires, which could be problematic for drivers outside bigger cities.

Run-flats also have some performance trade-offs, Cowger says. Since run-flat tires have reinforced sidewalls, the tires are heavier and have a stiffer ride. They also have more rolling resistance, which can affect your fuel economy.

Since 2007, the use of run-flat tires has increased 156 percent (representing 25 models), but they are standard on only 12 percent of new vehicles, according to Edmunds data. Traditionally, carmakers use run-flat tires on sports cars, but in recent years they have started to use them for other cars, too. BMW, whose luxury cars have sportier tendencies, has made run-flat tires standard on nearly every model. Even the newer, decidedly non-sporty Toyota Sienna minivan has a model that comes with run-flat tires.

Tire Repair Kits

A tire repair kit, also called an inflator or mobility kit, takes up the least amount of space in a trunk because no spare is involved. Instead, you get a kit consisting of an air compressor and an integrated bottle of a thick, viscous sealant.

“This is one that often surprises new car buyers,” Berger says. Sometimes the dealer doesn’t tell the car buyer about the kit, “and when the customers go home, they start playing with their car, and they can’t find any kind of spare tire. And when they look in some hidden space, they find this little black box with a little cigarette-lighter extension cord to it. They think ‘What in the world is this?'”

In the event of a flat tire, you attach the inflator-kit hose to the valve stem of the tire. When you turn on the unit, it injects the sealant into the tire, and you then use the compressor to inflate the tire. The advantage of these kits is that you don’t have to jack up the car or remove the tire. There is an instructional video from GM that shows you how it’s done.

They’re a good alternative for people who might not have the muscle to change a flat, as it takes a certain amount of physical ability to break the lug nuts loose, take off the flat tire and put on the spare.

However, the repair kit has limitations of its own. It can’t be used to fix about 15 percent of punctures, Berger says.

The kit can only be used if the puncture is on the tire’s tread. If the puncture is on the sidewall and is larger than a quarter of an inch, you can’t use the kit. Finally, you can’t use the kit if the tire has separated from the wheel. In those cases you would have to call roadside assistance and have the vehicle towed to the nearest tire shop or dealer.

The experts also disagree on whether tires can be reused after a repair.

“Most automobile manufacturers deem the tire to be scrap once that sealant is applied,” Berger says. There’s a chemical interaction between that seal and the tire. And the material that’s left behind really can’t be cleaned effectively to ensure that it won’t hurt the tire, he says.

Conversely, GM’s Cowger says that the sealant material, which solidifies into a rubbery substance, can be cleaned out. The tire can then be repaired just like any other, provided that the puncture is in the tread, not the sidewall.

Jack Martz, manager of Stokes Tire Pros in Santa Monica, California, has worked with Edmunds long-term road test cars. He says that while the sealant material is a pain to clean out, his shop can usually repair the tire without any issue. But it’s critical that the material be thoroughly cleaned as soon as possible. If the sealant does not have the right anti-corrosive ingredients, it could damage the tire, wheel or tire-pressure monitoring system.

And after a repair, the car owner must buy a new bottle of sealant, since the repair kit only holds enough fluid for one application. In the case of the Chevrolet Cruze, this costs roughly $30 at the dealership.

Edmunds has only been tracking the use of repair kits since 2009, but the list of cars without a spare tire is growing quickly. According to data, there are now 65 more models that have repair kits than there were in 2009. That’s a 283 percent increase in cars using them as a spare tire substitute. Repair kits are standard in 25 percent of the current vehicles available and outnumber the cars that come with run-flat tires and full-size spares.

Automakers consider several factors as they make spare-tire decisions, including storage space, driver safety and environmental concerns, says Bridgestone’s Berger.

Space considerations: Modern vehicles have more computers and electronics than cars had in the past, and these things take up room. Since trunk space is at a premium, automakers look for what they can shrink, and the spare tire is a likely candidate. A smaller spare usually means more usable trunk space.

The environment: Berger says this refers to the factors of weight and waste. A smaller spare weighs less and in turn, helps the car get better fuel economy. A smaller tire also means less rubber in the manufacturing and less rubber that could potentially end up in landfills.

Safety: This consideration comes into play when the car owner actually needs to use the spare. “Often you find yourself in the worst possible place to change a tire,” Berger says. “Even in the best circumstances, most people haven’t done it before.” Berger says the driver has to find the jack, and correctly position and use it.  An improperly lifted car is fraught with hazards all its own. That’s why two of the spare-tire replacement options in modern cars do not require a jack at all.

The Future of Spares

In the immediate future, automakers will continue to look for ways to shed weight from their vehicles. It’s imperative to meeting increasing fuel economy requirements.

“Over the last two years at General Motors, we’ve moved more toward inflator kits as standard equipment,” Cowger says.

For those who prefer temporary spares, GM offers them as a dealer-installed option that typically costs about $100.

Truck buyers can rest assured that their vehicles will still come with full-size spares for the foreseeable future. “Consumers will continue to demand full-size spares for the type of work they do, towing trailers and going off-road,” Cowger says.

Send us mail