TPMS UPDATE: No Spare, No Problem

TPMS UPDATE: No Spare, No Problem

In addition to technical improvements that have made flats less likely, TPMS provides drivers with warnings of low air pressure, leaks and punctures.


Automakers are selling more cars with four wheels and tires instead of five to trim weight, boost gas mileage and save money.  Fortunately, fewer motorists need to change tires anymore. In addition to technical improvements that have made flats less likely, TPMS provides drivers with warnings of low air pressure, leaks and punctures. Often, that means the tire gets properly inflated or fixed before it goes flat or before damage occurs, resulting in a blowout.

The benefits of eliminating the spare are too big for most automakers to ignore. Engineers struggle to reduce a car’s weight by ounces, and getting rid of the spare tire is a way to shed up to 50 pounds. Generally, a 10 percent reduction in vehicle weight yields a 6 percent improvement in fuel economy – and spare tires and jacks are easy targets.

Gaining a 10th of a mile per gallon in federal fuel economy tests is important in meeting ever-expanding CAFE standards. Those pounds and ounces may allow an automaker to reach 29.5 mpg on a vehicle – which can be rounded up to 30 mpg on the window sticker.

The consumer benefits too; a 1 mpg difference in fuel efficiency may save more than $100 per year, according to the Department of Energy. If an owner drives 100,000 miles carrying around a spare tire they never use, it burns a lot of extra gasoline. Also, deleting the spare often provides more trunk space.
The cost savings to auto manufacturers are substantial: eliminating the spare saves at least $20 per car. In the 2012 model year, approximately 15 percent of new cars came without spare tires. With the exception of pickups and SUVs likely to be driven off-road, the trend for most vehicles is to eliminate the spare. In most cases, a spare won’t even be offered as optional equipment – and there may not even be a place to put a spare.

When tire failures do occur, drivers increasingly rely on roadside assistance services to take care of the problem. With help easily available through cell phones, many people simply choose not (or don’t know how) to deal with flat tires – even if they have a spare. Drivers may still fear being stranded, but the almost universal use of cell phones has made that much less likely – whether or not they have a spare.
So how is the owner of a no-spare car supposed to deal with a flat? No matter if the car has run-flats or fix-a-flat, the owner must obey their TPMS system and either re-inflate or repair the flat. If they choose to ignore the problem, they will definitely be stranded.

Many expensive cars are opting for run-flat tires, which can be driven at moderate speeds for 50 miles or so with a puncture. The reinforcement built into run-flats supports the weight of the car and is designed to allow a driver to find a safe spot to stop, rather than being stranded in an unsafe place or on the side of a highway. But, since run-flats are limited to 50 miles after they lose air pressure, if a motorist is too far from civilization, they may not help much.

Flat-Fixing Kits
As an alternative, many OEMs are replacing spare tires with “mobility kits” designed to fix most flats. These consist of a can of sealant that is injected through the valve stem to plug the puncture, and a small electric compressor to reinflate the tire.

Tire mobility kits typically weigh less than six pounds, compared to 30 for a temporary spare and 50 or more pounds for a full-size spare. Unfortunately, they, too, have drawbacks. The kits generally only work on punctures of 1/4-inch or less in the tread or shoulder areas of the tire. Blowouts, cuts, cracks and sidewall damage that potholes frequently inflict on low-profile tires cannot be repaired by the kits. OEM sealants are typically approved for use with TPMS sensors. But, long-term exposure can lead to damage.

Using these kits is pretty simple: plug the unit into a 12-volt power outlet (cigarette lighter), and connect the air hose from the compressor to the tire valve. Once the sealant tank is flipped up, the compressor re-inflates the tire and fills the tire with a latex-based liquid sealant, which seals the puncture. This usually takes five to seven minutes. Then, the tire can be used at a maximum speed of 50 mph for up to 125 miles.
The instructions on most kits suggest driving four or five miles and then rechecking the inflation pressure with the built-in pressure gauge. Standard pencil, dial or digital tire pressure gauges should not be used because they can be ruined by the sealant. After using the mobility kit, the sealed tire should be driven to the nearest tire shop and inspected to determine whether it can be permanently repaired or must be replaced.

Another potential problem for consumers is the limited life span of the sealant. Sealant canisters all have “use by dates,” which owners are advised to check. Depending on the kit, the sealant canister typically should be replaced after four or five years. It’s not hard to imagine that sealant canister “use by dates” will probably be checked by most consumers about as often as they check spare tire inflation pressures.
In addition to their OE fitments, both the Continental and Dunlop kits are aimed at consumers who don’t have a spare, but don’t want to purchase replacement run-flat tires. They weigh a little more than five pounds each, can easily be stored in the trunk of a vehicle, and both also can be used to check and monitor tire pressure through a built-in compressor and tire gauge.

Stop & Go International offers an alternative tire mobility kit that combines a compressor with a Pocket Tire Plugger, necessary hand tools and mushroom-shaped rubber plugs to repair punctures. The mushroom head of the plug is designed to seal the puncture and allows for the resumption of normal highway speeds. By avoiding spray-in tire sealant, Stop & Go claims to avoid the potential for damage to TPMS sensors and the necessity for a tire dismount.

However, the advisability of using a plugged, rather than a properly repaired tire, is a real issue.

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