DETROIT • Don't know how to change a punctured car tyre?
Even if you knew, you are unlikely to be able to apply that knowledge. The spare tyre is no longer commonly found in many new cars, joining other items such as the metal ignition key, ashtray and vent window that swings open.
Already, nearly a third of this year's models in the United States do not come outfitted with the save-the-day spare as standard equipment, according to a study by the American Automobile Association (AAA).
In truth, the phasing out of the spare tyre has been happening, if gradually, for years.
The elimination by carmakers is not entirely an abandonment of good sense or an extreme example of cost-cutting.
In fact, it can benefit drivers.
The primary goal is weight reduction, a crucial factor in meeting fuel-economy standards.
Removing the substantial amount of rubber and steel of up to 18kg, according to industry experts, along with a jack and lug wrench, is a big win for engineers.
But the disappearing spare can cause headaches. AAA said that last year, it answered roadside assistance calls from 450,000 members whose cars did not have spares.
The freedom to eliminate this item is largely possible due to developments in tyre construction technology.
An increasingly popular alternative is the so-called run-flat design which most new BMW models use.
Intended to make roadside tyre changes unnecessary, this solution employs a reinforced tyre sidewall that typically lets the driver continue for 80km at up to 80kmh after air pressure is lost.
But it can be more costly as it may be necessary to replace, rather than simply patch, a damaged tyre, and replacements are typically priced US$25 (S$34) to US$50 higher than a conventional design.
An alternative is the self-sealing tyre, an older solution reappearing in modern form on the battery-powered Chevrolet Bolt, where reduced weight translates to more kilometres a charge.
Designed solely as an electric vehicle, the Bolt has no provision for carrying a spare. According to Michelin, which supplies the car's Energy Saver A/S Selfseal tyre, the extra cost of a self-sealing tyre - which can continue down the road even with a nail in the tread - is about US$33 compared with conventional tyres of the same size.
But some models are losing the spare without the benefit of run-flat or self-sealing rubber, instead including conventional tyres and a leak repair kit packaged in an aerosol can or used in conjunction with a small air compressor powered by the car's battery.
But such kits have limited abilities to deal with any road hazard more serious than a nail hole in the tyre's tread section.
A larger tear in the tyre - something that can happen when modern low-profile tyres meet a pothole - or damage to the sidewall or wheel rim will not be fixed by a leak kit.
The sealants, which are usually one-time-use devices, have a finite shelf life of four to eight years, AAA said, and cost about US$40 to replace.
Not surprisingly, independent online retailers have emerged to fill the hole.
But buyers should make sure to compare prices with the dealership and determine that there is a storage spot in the car where the tyre can be secured.
As designers work to smooth airflow under the car for fuel-economy improvements, those compartments are vanishing too.
The disappearance of the spare tyre might be more than just an exercise in efficiency. It may be a sociological statement.
A survey by AAA found that some 20 per cent of drivers do not know how to change a flat tyre and, with the rise of roadside assistance coverage for new cars, that number is unlikely to shrink.