A customer surprised by a flat tyre may want a mechanic to come out and replace the damaged tyre with a spare tyre. Spare tyres in many vehicles are intended only to be used temporarily until you purchase a new tyre (or set of tyres). Spare tyres are a great solution when you find yourself with a flat tyre, so you should always keep a spare tyre in your vehicle. If you don't feel comfortable putting the spare tyre on, you should call a mechanic to do it for you. Cars need four tyres to operate. Most vehicles come equipped with a spare tyre in the trunk, just if one of your regular tyres blows or goes flat. If a tyre loses its pressure, you can use the spare tyre as an interim tyre until you get it replaced. Spare tyres may be compact, or doughnut spare tyres that are smaller than regular tyres, and for safety concerns, shouldn't be driven for long periods of time, but they allow you to get your car mobile again after a flat tyre.