Spare tire

A spare tire (or stepney in some countries) is an additional tire (or tyre - see spelling differences) carried in a motor vehicle as a replacement for one that goes flat, has a blowout, or has another emergency. "Spare tire" is generally a misnomer, as almost all vehicles actually carry an entire wheel with a tire mounted on it as a spare rather than just a tire, as fitting a tire to a wheel would require a motorist to carry additional, specialized equipment. However, some spare tires ("space-saver" and "donut" types) are not meant to be driven long distances. Space-savers have a maximum speed of around . 
When replacing a damaged tire, placing the compact spare on a non-drive axle will prevent damage to the drivetrain. If placed on a drivetrain axle, the smaller-diameter tire can put stress on the differential causing damage and reducing handling.
History.
The early days of motor travel took place on primitive roads that were littered with stray horseshoe nails. Punctures (flat tires) were all too common, and required the motorist to remove the wheel from the car, demount the tire, patch the inner tube, re-mount the tire, inflate the tire, and re-mount the wheel. 
To alleviate this time-consuming process, Walter and Tom Davies of Llanelli, Wales, invented the spare tire in 1904. At the time, motor cars were made without spare wheels. The wheel was so successful that the brothers started their own company, Stepney Spare Motor Wheel Limited, (named after the location of their workshop on Stepney Street In Llanelli) and started marketing the wheel in Britain, Europe, and the British Empire and colonies. The word "stepney" is sometimes used interchangeably in countries that were once part of the British Empire such as Pakistan, Bangladesh, India, and Malta.
The first to equip cars with an inflated spare wheel-and-tire assembly were the Ramblers made by Thomas B. Jeffery Company. The Rambler's interchangeable wheel with a mounted and inflated spare tire meant the motorist could exchange it quickly for the punctured tire that could then be repaired at a more convenient time and place.
The pre-mounted spare tire and wheel combination proved so popular with motorists that carrying up to two spare tires became common. Automakers often equipped cars with one or dual sidemounts. The spares were mounted behind the front fenders as they blended into the running boards (a narrow footboard serving as a step beneath the doors).
In 1941, the U.S. government temporarily prohibited spare tires on new cars as part of the nation's World War II rationing strategy, which led to quotas and laws designed to force conservation, including rubber that was produced overseas and difficult to get. A similar ration prohibition was also implemented by the U.S. during the Korean War in 1951.
Usage in the 21st century.
Contemporary vehicles may come equipped with full-size spares, limited use minispares, or have run-flat capability.
Storage.
Spare tires in automobiles are often stored in a spare tire well – a recessed area in the trunk of a vehicle, usually in the center, where the spare tire is stored while not in use. In most cars, the spare tire is secured with a bolt and wing-nut style fastener. Usually a stiff sheet of cardboard lies on top of the spare tire well with the trunk carpet on top of it to hide the spare tire and provide a pleasant look to and a flat surface for the trunk space.
Other storage locations include a cradle underneath the rear of the vehicle. This cradle is usually secured by a bolt that is accessible from inside the trunk, for security. This arrangement has advantages over storing the tire inside the trunk, including not having to empty the contents of the trunk to access the wheel and this arrangement may also save space in some applications. However it has disadvantages because that tire gets dirty, making the act of changing the tire more unpleasant and the mechanism may also rust on older cars, making it difficult to free the spare. The cradle arrangement is usually only practical on front wheel drive cars, as the cradle would get in the way of the rear axle on most rear or four wheel drive cars. A similar arrangement is also often found on trucks where the spare is often stored beneath the truck bed.
Many sport utility vehicles (SUVs) and off-road vehicles have the spare wheel mounted externally – usually on the rear door, but others may mount them on the roof, the side, or even on the bonnet (hood).
In mid-engined and rear-engined cars, the spare tire is generally stored in the front boot.
Some vehicles stored the spare tire in the engine bay, such as the Renault 14, First generation Fiat Panda and older Subaru vehicles, such as the Subaru Leone.
Vehicles like the Volkswagen Beetle used spare tires for ancillary purposes such as supplying air pressure to the windscreen washer system.
Many models of Bristol cars - those from the 404 of 1953 to the Fighter of 2004 carried a full-size spare wheel and tire in a pannier compartment built into the left-hand wing. This not only increased luggage space and allowed easy access to the spare without having to unload the trunk but improved weight distribution by keeping as much mass as possible within the wheelbase and balancing the weight of the battery, mounted in a similar compartment in the right-hand wing.