It may seem obvious, but it cannot be used as a spare tire. These donuts are designed as a temporary measure for flat tires. Therefore, the specifications will not be the same as normal tires. That doesn't mean it will collapse after a mile, but it has its limits.
Most donuts are designed to last 50 miles, and in some cases up to 70 miles. That doesn`t mean per use, that`s an overall usage mark. These tires should also not be driven faster than 50 miles per hour. So, stay off the highways, stick to city roads, and keep it under 50 mph. You`ll be able to make it safely to a repair shop or a tire store to get your tire inspected, fixed, or replaced in one piece.
Just by looking at them, it should become readily apparent why donut spares are rated for such little use. For starters, they aren`t very wide, so the vehicle won`t handle as well as it does with a normal tire. The tread usually isn`t as robust as the other tires, which could lead to some hydroplaning as well. The most obvious reason is the donut just isn`t built for continual use. They`re made to make sure you can get your car off the roadway safely and to the nearest mechanic or tire store. When you put it to the test, you might find yourself needing a spare for your spare.
Prolonged driving of a spare tire can cause significant damage to other parts of the car. It is not the same size as regular tires, which affects the balance of the car. This puts an unnecessary burden on your other three tires. Tire mismatches can also be a problem for vehicle differentials.