Four ways cars deal with a flat
Cars no longer all come with a conventional spare. Depending on the vehicle, the maker may supply a full-size spare, a smaller temporary one, run-flat tires, or a sealant and inflator kit. Each approach is a trade-off between weight, space, cost and convenience. Knowing which one your car has, before you need it, is one of the most practical things you can do.
Full-size spare
This is a complete wheel and tire that matches (or closely matches) the four on the car.
Advantages: you can drive normally on it, at least for a while, and it avoids many limits on speed and distance. If it is identical to the other tires, it can even be included in a rotation.
Drawbacks: it takes more space and adds weight. It also ages even when unused, so its pressure and condition need checking from time to time. Some “full-size” spares use a plain steel wheel that differs from the alloy wheels on the car, and some are a different but compatible size. Check the manual for any restrictions.
Space-saver (temporary) spare
A space-saver, sometimes called a compact or “donut” spare, is a smaller, narrower wheel and tire made to save weight and trunk space.
Advantages: compact, light and quick to fit.
Drawbacks: it is designed for short-term use only. A speed limit is usually printed on the spare itself, often around 50 mph (80 km/h), and the maker may also limit the distance. Handling and braking can differ because it is not the same size as the other tires, and some safety systems may behave differently. It often needs a higher pressure than normal tires, noted on the tire or in the manual. Replace it with a proper tire as soon as possible. The procedure for fitting one is described in the article on changing a flat.
Run-flat tires
Run-flat tires have reinforced sidewalls, or an internal support, that let the tire carry the vehicle for a limited distance after losing pressure.
Advantages: you can often keep driving to a safe place or a tire shop rather than changing a wheel at the roadside, which can be helpful in unsafe locations. It also means no need to carry a spare, which frees up space.
Drawbacks:
- The limit is typically a modest distance at a reduced speed. A commonly quoted figure is about 50 miles at up to 50 mph, but the actual limits vary by tire, so check the manual and the tire markings.
- Run-flat tires are usually firmer, and the ride can feel harsher.
- They are often more expensive to replace and may not always be repairable after being driven flat, since the sidewall can be damaged unseen.
- Cars that rely on them need a working pressure monitoring system, because a run-flat can feel normal when it has lost air. A dashboard warning is your cue to check, and the explanation is in the article on the tire pressure monitoring light.
Run-flat tires can be recognized by a marking on the sidewall, though the symbols vary by manufacturer. The article on sidewall markings gives more on this.
Sealant and inflator kits
Many newer cars come with a small compressor and a bottle of liquid sealant instead of a spare.
How it works: the sealant is injected through the valve, and the compressor reinflates the tire. As you drive, the liquid is spread by the rotation and can seal small punctures in the tread.
Advantages: very light, cheap and takes little space, and it does not involve jacking the car.
Drawbacks:
- It works only on small punctures in the tread area. It will not fix a large cut, a damaged sidewall or a tire that has separated from the rim.
- Sealant has an expiry date, and the bottle needs replacing after use or when it runs out of date.
- Some tire shops need to clean the sealant out of the tire, and it may complicate a later repair.
- It is a temporary measure. After using it, the manual will usually say to drive a short distance and have the tire inspected or replaced.
- It does not help if you are stranded with a tire that cannot hold air.
How to choose or check what you have
If your car already came with one of these, the practical questions are:
- Where is it? Many people have never looked in the trunk floor or under the car. It is worth finding out now.
- Is it in working order? Check a spare’s pressure at intervals and confirm the jack and wrench are present. Check the sealant’s expiry date and whether the compressor runs.
- What are its limits? Note the speed and distance restrictions for a temporary option.
- Do I need something extra? If your car has only a sealant kit, you might consider a plug kit or a roadside assistance plan, which are explained in the article on roadside assistance. A tow may be needed in some situations, as covered in how towing works.
Changing the type of tire, such as moving from standard to run-flat tires, can affect ride, cost and the systems on the car, so ask a tire technician and consult the manual first. The right option varies by vehicle, climate and where you drive, so no single choice is best for everyone.