Spare Tires in New Cars: What You Need to Know

You're late to work, it's raining and the kids need to be dropped off at school. That's when it happens: You hear a loud pop or you see the tire pressure light come on. As you inspect the tires, you notice the culprit: a nail.

"No big deal," you say to yourself. "I have a spare."

But when you reach into the trunk, there is no spare tire. In its place, you find a small device with a gauge and a power plug. It's a tire repair kit, and at the rate things are going, learning how to use one of these may be more valuable than learning how to change out a spare.

Decades ago, nearly every car came with a full-size spare tire. But fuel economy requirements, trunk space considerations and the dangers involved in setting up a jack on the side of a road prompted automakers to shift toward smaller, temporary spares. Today, some vehicles have no spare at all. That doesn't mean carmakers are stranding you, however. Your car may be equipped with run-flat tires. Or, in place of the spare, you'll find that tire repair kit.

The worst time to find out about the different types of spares is when you have a flat. It's better to learn about the pros and cons of various spares during your search for a new car. It can be as easy as popping the trunk to see what's there. Depending on how strongly you feel about the spare type, its presence or absence might influence your decision on which car to buy. Let's take a closer look at each spare tire type and how they've changed over the years. We'll talk first about the ones people might be least familiar with.

Tire Repair Kits

Many automakers are favoring tire repair kits over spares because they take up very little space.

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 with an attached hose and an integrated bottle of a thick, sticky sealant.

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. Then you use the compressor to inflate the tire. The advantage of an inflator kit is that you don't have to jack up the car or remove the tire. This instructional video from Ford shows you how it's done.

Repair kits are a good alternative for people who might not have the muscle to change a flat. It takes a certain amount of physical ability to break the lug nuts loose, take off the flat tire and put on the spare.

The repair kit has a few limitations, however. The kit can only be used if the puncture is on the tire's tread and is not larger than 4 millimeters, about the size of a pencil eraser. If the puncture is on the sidewall or is larger than 4 mm, 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 need 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," said Kurt Berger, manager of consumer products engineering for Bridgestone, which provides tires for many automakers. 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 said.

Conversely, Dave Cowger, global tire subsystem leader at General Motors, said 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 the puncture is in the tread, not the sidewall.

Jack Martz, manager of Stokes Tire Service in Santa Monica, California, has worked with Edmunds long-term road test cars. He said that although 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 holds only enough fluid for one application. This costs roughly $30 at the dealership.

Edmunds has been tracking the use of repair kits since 2009, and there are now 110 models carrying them compared to just 23 in 2009. That's a 378 percent increase. Repair kits are standard in approximately 30 percent of 2017 vehicles, outnumbering the cars that come with run-flat tires and full-size spares.

Run-Flat Tires

Most BMWs come standard with run-flat tires.

Vehicles equipped with run-flat tires have no spare or repair kit. 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 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 said.

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. Also, not every tire shop carries run-flat tires. That could be a problem if you live or travel outside bigger cities.

Run-flats also have some performance trade-offs, Cowger said. Because run-flat tires have reinforced sidewalls, the tires are heavier and have a stiffer ride. They also have more of what's called rolling resistance, which can affect your fuel economy.

Since 2007, the use of run-flat tires has increased 225 percent (representing 36 models). The number seems high, but they are the least common alternative to regular tires. They are standard on just 14 percent of 2017 vehicles, according to Edmunds data. Traditionally, carmakers have used run-flat tires on sports cars, but in recent years they have started to use them for other cars, too. Even the decidedly non-sporty Toyota Sienna minivan has a model that comes with run-flat tires.

BMW, whose luxury cars have sportier tendencies, has made run-flat tires standard on nearly every model.

"Since many people do not know how to change a tire, we believe there is a significant safety advantage because they allow the driver to get their vehicle either to a place that can address the flat tire or to a safe place to await help," said Dave Buchko, a BMW spokesperson.

Temporary Spares

Temporary spare tires are the most common and offer the best balance between size and usability.

Temporary spares or "donuts" are the most common choice for automakers. They offer the best balance between size and usability and come standard on 53 percent of 2017 models. That's a 15 percent increase from 2007, according to Edmunds data. 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," Berger said. "The carmaker and tiremaker 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.

Full-Size Spares

SUVs and trucks are two of the few holdouts in the shift away from full-size spare tires.

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 reduce vehicle weight to improve fuel economy and make the spare easier to install, according to Tire Rack.

Although they are the biggest and heaviest of the spares, full-size tires offer virtually no performance loss. If you have a matching spare, you can 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.

There has been a 38 percent decrease over the past decade (accounting for about 50 fewer models) in vehicles that come equipped with full-size spares. Except for trucks or larger SUVs, full-size spares have disappeared from the majority of passenger cars. The full-size spare is standard on 22 percent of 2017 vehicles.

A Phone Instead of a Spare?

In the immediate future, automakers will likely keep looking for ways to shed weight from their vehicles. We've already seen BMW embrace run-flat tires. GM has moved more toward tire repair kits in many of its vehicles.

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," said GM's Dave Cowger.

If the thought of a missing spare fills you with anxiety, here's a reality check: Most people reach for the phone, rather than the spare, 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," Cowger said. "Because of the convenience of roadside assistance, many of them make that call even if they have a spare."