The small spare tires that are provided with most cars are only intended as temporary spares. As a general rule, they should be driven no more than 70 miles, and the car should not exceed 50 miles per hour.
Compact spares are not as durable as standard tires. They have limited puncture resistance and poor cornering ability. They are also narrower than standard tires, therefore providing less traction. This can lead to longer stopping distances and poor handling. In addition, the smaller tire can put a strain on the car's differential.
A full-sized spare doesn't present the same risks as a compact spare, but the difference in wear between newer and older tires can cause the car to handle poorly. For this reason, most manufacturers suggest replacing tires in pairs.