From a 1934 tyre sold as semi-bulletproof to the disappearing spare, each option covers something different after a puncture.
A run-flat tyre keeps a punctured car moving, typically about 50 miles at around 50 mph, using a stiff sidewall, a sealing lining or a wheel insert. It costs more, weighs more and rides harder. New cars drop the spare to save cost, weight and waste, leaving space-savers or repair foam.
In 1934, Michelin sold a tyre that ran on a foam lining after a puncture, and advertised it as "semi-bulletproof": the buyers were militaries and the operators of bank armoured cars. Decades on, the run-flat survives in a few showrooms while many new cars have lost their spare entirely.
How do run-flat tyres work?
A run-flat tyre is built to resist the effects of deflation after a puncture, so the car can keep moving at reduced speed for a limited distance. Michelin first developed the idea in the 1930s, and run-flats reached the public market in the 1980s. Three basic technologies exist, and they work in quite different ways.
Behind all of them is the plain job of any tyre. It carries the weight of the vehicle down to the road by way of the axle and wheel, and the flexible cushion of air inside absorbs shocks. A run-flat has to go on doing that job when the air has gone. The basic benefit is continued mobility after a loss of pressure, whether from an ordinary puncture or from a deliberate hostile act such as a bullet strike at speed, and the yardsticks are how far and how fast the vehicle can escape and how well it can still be steered.
Self-supporting: the stiff sidewall
The self-supporting design is the one most drivers meet. A stiff sidewall holds the car up when the air escapes and protects the rim from damage, which removes the need for a spare. Self-supporting tyres have become standard fare on light trucks and passenger cars, and the usual allowance is 50 miles (80 km) at about 50 mph (80 km/h).
Its commercial roots go back to 1935, when a tyre with a fabric inner tyre was sold as protection against blowouts, then a common and dangerous event. In 1958 Chrysler worked with Goodyear on Captive Air run-flats, which had an interlining bearing the load. Dunlop followed in 1972 with its Total Mobility Tyre, later called Denovo, as a "fail-safe" wheel and tyre system. It became optional on the Rover P6 3500 in 1973 and evolved by 1983 into the TD/Denloc, which was standard across the whole Austin Metro range.
More recently, Bridgestone and Pirelli run-flats have been supplied on some new BMW cars, which pitched them both as a safety feature and as a way to avoid carrying a spare. The Pressure Zero Tire, a design aimed at heavy military pick-ups, was pioneered by American Engineering Group for US Special Forces.
Self-sealing: the tyre that mends itself
A self-sealing tyre contains an extra lining that closes up a small hole made by a nail or screw. Air loss is stopped from the start, so the tyre is either permanently self-repairing or loses air only very slowly. Retrofit sealants work in a similar way. They are injected through the valve, and rotation spreads the compound over the inner surface to form a sealing lining.
Auxiliary-supported: the insert on the wheel
The third approach adds a support ring or insert to the wheel itself, which carries the vehicle's weight after pressure is lost. It can bear heavy loads for long distances at high speed, which is why it is the usual pick for military vehicles, vehicles protecting senior executives, and the armoured vehicles that governments, aid groups and private contractors run in conflict zones.
The 50-mile, 50-mph envelope is therefore a limit and not a target.
Are run-flat tyres worth it?
For many buyers the answer has been no, mainly because of cost and limits. The tyres can cost more than double a comparable standard tyre, they cannot be driven flat if the damage is in the sidewall, which is a common cause of flats, and they cannot normally be driven faster than 50 mph. Those limits lower the value of the extra expense.
The penalty also shows up in everyday driving. Self-supporting run-flats carry a weight penalty of 20 percent to 40 percent compared with standard tyres, and the thicker sidewall raises rolling resistance, which hurts fuel economy. The stiff walls let more road imperfections filter through to the rim and the car, so the ride is harsher.
There is an offset. The weight penalty of each tyre is usually more than made up for by the fact that the car no longer carries a spare wheel or the equipment to swap it in.
The market verdict
The numbers explain why run-flats stayed rare. They made up less than 1 percent of replacement tyre sales in the United States in 2005. A 2006 forecast expected armoured-vehicle makers to boost demand, but growth was slow, and Michelin stopped developing its PAX System, though it continues to make replacements for the foreseeable future. A Michelin study published in 2008 found that 3 percent of drivers worldwide want run-flat tyres.
American Honda said its last run-flat models would be the 2009 Odyssey Touring and Acura RL. Only a handful of volume manufacturers now offer them as standard, and only on some models. BMW fits more run-flats at the factory than anyone else. The Goodyear Kevlar-lined run-flats on Barack Obama's presidential limousine, nicknamed the Beast, are the rare high-profile appearance.
Can you repair run-flat tyres?
Not always, and often not safely. A run-flat that has been driven beyond its limits may leave the wheel damaged, and the repair may be impossible or unsafe, particularly after a puncture in the sidewall or along the tread's edge. Sidewall damage also rules out running the tyre flat in the first place.
The 50-mile, 50-mph envelope is therefore a limit and not a target. Depending on the vehicle, the tyre design and the driving surface, the distance a run-flat can cover while flat at restricted speed runs from 25 to 200 miles (40 to 322 km). Under ideal conditions the speed and range are still limited.
Why do new cars drop the spare wheel?
Cost, weight and waste. Leaving out the spare cuts the price of the vehicle, reduces fuel consumption and cuts production waste, so newer vehicles often come without one. In its place, a car may rely on run-flat tyres or carry a can of repair foam.
The shift began with a half-measure. The limited-use spare, variously called a space-saver, temporary, donut or compact type, arrived in the late 1970s to cut cost and weight and to conserve the room a full-size spare takes. By 2017 temporary spares were standard on 53 percent of that year's models in the US. A space-saver typically weighs 7 kg (15 lb) less than a full-size wheel, though in some cars it saves little or no space, and carrying the full-size wheel home once the space-saver is fitted is a problem.
The full-size spare
Strictly, the phrase spare tyre is a misnomer. Almost all vehicles carry a whole wheel with a tyre mounted on it, because fitting a tyre to a bare wheel at the roadside would demand specialised equipment. The wheel may match the road wheels or sit on a plain steel rim.
Full-size spares survive where a limited-use spare would be a poor fit. SUVs and light trucks are usually given one because their higher centre of gravity would be hurt by a space-saver. A limited-use spare may also be unsafe on a fully loaded truck or one towing a trailer. Because of its size, the full-size spare is often mounted outside, such as on the rear door of an SUV, and occasionally on the bonnet.
Spare wheel versus puncture kit: what each gets you home on
| Option | What it offers | Drawbacks |
|---|---|---|
| Full-size spare | Same as the other wheels | Takes room; often mounted outside on SUVs |
| Space-saver spare | Limited distance, maximum around 50 mph (80 km/h) | Stability control and traction control do not work properly; may compromise braking and handling |
| Self-supporting run-flat | About 50 miles (80 km) at around 50 mph (80 km/h) | 20 to 40 percent weight penalty, harsher ride, higher cost |
| Repair foam | A can that treats the punctured tyre rather than carrying the car | Often fails on larger punctures; useless in a blowout |
What should you know about a space-saver spare?
A space-saver is smaller than the normal tyres, and its short life expectancy and low speed rating restrict it to limited distances. It is not meant for long journeys, and its top speed is around 50 mph (80 km/h).
The different size has effects on the car's electronics. Electronic stability control and traction control do not operate properly with a donut and should be switched off until the proper wheel is back on. A space-saver could also compromise braking, especially on cars without anti-lock brakes, and handling.
Where it goes on the car matters. Placing a compact spare on a non-drive axle prevents damage to the drivetrain. On a drive axle the smaller diameter can stress the differential, causing damage and reducing handling.
Repair foam is the lightest option, and its record is mixed: it often does not work on larger punctures, and it is useless in a blowout.
Where is the spare stored, and where did it come from?
Most spares live in a recessed well in the boot, often fixed by a bolt and wing nut. Some sit in a cradle under the rear of the car, which spares the owner from emptying the boot but gets the tyre dirty and can rust the mechanism, and which suits front-wheel-drive cars best. Many SUVs mount the spare outside. Cars with the engine in the middle or at the back usually keep the spare in the front boot.
Some makers have been inventive. The Renault 14, the first-generation Fiat Panda and older Subarus such as the Leone stored the spare in the engine bay, and the Volkswagen Beetle used its spare to supply air pressure to the windscreen washer. Every Bristol between the 404 (1953) and the Fighter (2004) had a full-size spare in the left wing, freeing luggage space and counterbalancing the battery in the right wing.
The spare itself has a Welsh origin. In the early days of motoring, roads were littered with stray horseshoe nails and punctures were common, forcing drivers to take the wheel off, demount the tyre, patch the inner tube and put everything back. Walter and Tom Davies of Llanelli invented the spare tyre in 1904 to end that chore, and their company was called Stepney Spare Motor Wheel Limited, after the street of their workshop, which is why a spare is still a "stepney" in places including Malta, India, Bangladesh and Pakistan.
The first cars with an inflated spare wheel-and-tyre assembly were Ramblers from the Thomas B. Jeffery Company. Spares grew so popular that carrying two became common. In 1941 the US government temporarily banned spares on new cars as part of wartime rationing, and it did the same during the Korean War in 1951.
Where this goes next
Where the run-flat idea remains strong is in demanding uses. Auxiliary-supported inserts stay the normal selection for military, protection and armoured vehicles. The usual yardstick for military and security vehicles is the Finabel standards.
