No spare wheel: repair kits, space-savers and self-sealing tyres

What is under the boot floor?

There used to be a simple answer to the question “how many tyres does a car need?”: as many as it has wheels, plus one. Today, in a new car (or one recently brought in from abroad), far from every driver will find a fifth wheel. According to a count by a British motoring club, in 2023 only a few per cent of new models on the British market had a spare wheel of any kind as standard, even a narrow space-saver.

Why? Because a wheel with a jack and a brace means extra kilograms, and every kilogram means fuel and CO2 that the car maker has to squeeze into ever stricter environmental limits. In hybrids and electric cars the space under the floor is also needed for the battery. The maker’s logic is understandable. It is just that the maker will not be the one standing by the roadside at night with a flat tyre.

A repair kit: what it can and cannot do

The principle is simple: sealant is poured into the tyre through the valve, a compressor inflates it, and the liquid spreads over the inside and closes the hole. One of the largest European motoring clubs names the limits of this method: punctures a few millimetres across and only in the tread area (it is better to leave a nail or a screw where it is). A repair kit is not a spare wheel but only a way to reach a tyre fitter with a small puncture in the tread.

Now recall how a tyre’s acquaintance with a Ukrainian pothole usually ends. A sidewall cut, a bulge, a damaged bead, or a tyre “chewed up” by the wheel. A can of sealant will not help with any of these kinds of damage. It is like a plaster from the car first-aid kit: fine for a scratch, laughable for a fracture. And a sidewall cut in a radial tyre is exactly a fracture, with no room for compromise.

Three more things that the sales assistant usually keeps quiet about:

• After sealant: no more than 80 km/h and straight to a tyre fitter, not “I will drive on it for another week, it is holding, isn’t it”.
• A tyre that has had sealant put in is, as a rule, no longer repaired but replaced: the same motoring club writes that it cannot be repaired even at a garage, and the kit maker requires the tyre to be replaced as soon as possible. And that “cheap” can all too easily turn into the price of a new tyre.
• Sealant can clog the port of the pressure sensor on the valve, and then you have to buy a sensor as well (about tyre pressure sensors). A mechanic I know asks to be told about sealant before he touches the wheel. What traces of sealant do to fitting and balancing is covered in the article on inspecting part-worn tyres.

A space-saver: 80 km/h and no further than the tyre fitter

A “space-saver” is a narrow, lightweight spare wheel (the size of its tyre usually begins with the letter T) that takes up less room under the floor than a full-size one. On a narrow space-saver you must not drive faster than 80 km/h: it is written on the wheel itself, because Ministry of Infrastructure Order No. 549 of 2013 requires a warning sign limiting the speed to 80 km/h to be put on the tyre of such a wheel. And eighty is a ceiling on dry tarmac, not a cruising speed. For a space-saver that the car maker did not provide for, the same order sets an even lower limit: 60 km/h.

Why so strict? A space-saver tyre is designed only for a short, slow trip, the tread is shallow, and the pressure is, as a rule, 4.2 bar, almost twice that of an ordinary wheel. The contact patch is, roughly speaking, the load on the wheel divided by the pressure: double the pressure means a patch almost half the size. One of the four wheels grips the road noticeably worse than the others, and the car brakes and turns in unevenly. Hence some simple rules:

• Check the pressure in the space-saver (the figure is on the tyre or in the handbook) every month and without fail before a long journey: it lies idle for years, and a flat spare is no spare.
• Fit it the way your car’s handbook says. For front-wheel-drive cars it is often advised to put the space-saver on the rear axle and to move a full-size wheel to the front.
• Only as far as the nearest tyre fitter, not on a long journey (and only one space-saver at a time: that is also a requirement of Order No. 549).

And what about “five winter tyres”? I am not withdrawing that advice: if the car has a full-size spare wheel, it should be a winter tyre, like the other four. Nobody changes a space-saver with the season, and even the same order under which studded tyres are fitted to all wheels, the spare included, makes an exception precisely for a “temporary-use spare wheel”. So in winter on a space-saver, drive slower still and more carefully still.

Self-sealing tyres and run-flats

Now for what has really changed for the better. A self-sealing tyre has a layer of sticky sealant applied to the inside, under the tread: it wraps around a nail and closes the hole. According to one of the large manufacturers, such a layer closes tread punctures up to 5 mm in diameter and copes with about 80% of punctures. It is useful, but like a repair kit it works only in the tread: it does not protect the sidewall. After a puncture a fitter still has to inspect the tyre, and such tyres are repaired using a separate technique. The same goes for “quiet” tyres with a layer of foam inside: to put on a patch, the foam around the hole has to be cut out. The fitter has to know what is inside the tyre. And what can be repaired after a puncture at all is a conversation of its own.

Run-flat tyres let you drive on after a loss of pressure, as a rule up to 80 km at a speed of up to 80 km/h, and car makers fit them only together with a tyre pressure monitoring system (otherwise the driver would not notice that he is driving on a flat tyre). Life has met one of the two conditions under which run-flat tyres would stop being a niche product: since November 2014 all new passenger cars registered in the EU have been fitted with a tyre pressure monitoring system. As for the second, the price, compare for yourself in the price list of any tyre fitter.

What to carry with you

For Ukrainian roads, where a tyre is threatened not only by a nail but also by a pothole with a sharp edge of tarmac, I advise the following:

• If there is room in the recess under the floor, buy a space-saver, or better still a full-size wheel. A spare can, as a rule, be bought separately: choose it by the handbook, not “by eye”.
• Do not throw the repair kit away: as a second line of defence against a nail in the tread it will do, and the compressor will also come in handy when a wheel is losing air slowly but the sidewall is intact. Check the date on the can: sealant also has a shelf life.
• A pressure gauge, a jack and a wheel brace: it is quite possible that they disappeared from the car together with the spare.

To sum up: a repair kit does not save you from a cut sidewall, a bulge or a damaged bead, so on Ukrainian roads a full-size spare wheel is still the most reliable option, a space-saver is a sensible compromise, and a repair kit is a backup plan for the backup plan. How much boot space you are willing to give up for peace of mind on a road at night is for you to decide.

All articles in this section: Tyre care