A spare tire is an additional tire

Tire

A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

 (or tyre - see spelling differences

American and British English spelling differences

One of the ways in which American English and British English differ is in spelling.-Historical origins:In the early 18th century, English spelling was not standardized. Differences became noticeable after the publishing of influential dictionaries...

) carried in a motor vehicle

Motor vehicle

A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...

 as a replacement

Spare part

Spare Parts may also refer to:* Spare Parts , by Status Quo* Spare Parts is a Doctor Who audio drama.* Spare Parts , by Servotron* "Spare Parts" , by Bruce Springsteen* "Spare Parts"...

 for one that goes flat

Flat tire

A flat tire is a deflated pneumatic tire, which can cause the rim of the wheel to ride on the tire tread or the ground potentially resulting in loss of control of the vehicle or irreparable damage to the tire and wheel....

, a blowout

Blowout (tire)

A blowout is a rapid loss of inflation pressure of a pneumatic tire leading to an explosion. The primary cause for this is encountering an object that cuts or tears the structural components of the tire to the point where the structure is incapable of containing the pressurized air, with the...

, or other emergency. Spare tire is generally a misnomer

Misnomer

A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...

, as almost all vehicles actually carry an entire wheel as a spare, as fitting a tire to a wheel is very difficult without specialised equipment, and is not practical in an emergency. However, some spare tires ("space-saver" and "donut" types) are not meant to be driven long distances and most of them have maximum speed of around 50 mi/h.
The early days of motor travel took place on primitive roads that were littered with stray horseshoe

Horseshoe

A horseshoe, is a fabricated product, normally made of metal, although sometimes made partially or wholly of modern synthetic materials, designed to protect a horse's hoof from wear and tear. Shoes are attached on the palmar surface of the hooves, usually nailed through the insensitive hoof wall...

 nails.