I recently purchased four new tires for my vehicles. I know that some car owners save at least one for emergencies as a spare. I didn't because we found a brand new tire in the compartment so there was no need to save even the best of the four. I just assumed the spare was a donut tire and didn't bother to look. Hindsight - I could have bought three new tires and used the spare and saved some money and kept the best of the four for the spare. Do you save the old tires from your vehicle(s)?
Tires should always be replaced in at least 2. No reputable tire shop would let you buy just 3 new tires!! Myself, I do keep one tire for a spare. The reason is, the small temporary tires that come with the car are restricted to about 90km/h and can only be used for a short distance. I drive on the highway a lot so I keep a full sized tire in my truck at all times. You only really need to do this once though, because your spare doesn't really wear out. So to make a long story short yes I save them, but only one tire one time only.
You can do neat things with used car tires. A friend was recently telling me about their potato patch. They began with one tire on the ground. Filled it with dirt and planted some potatoes. Then as the potatoes grew, they continued adding tires and dirt, so that made a big tall tube. What happened was, it set the potatoes clear from the bottom to the top. So when it was time to harves, they tored it apart and it was chock full of potatoes! Cool