If you buy a new car that require the same size of tires and if your extra set of tires is in good condition, then it might be good to save them. It does however require that both these are fulfilled. If the extra set are winter tires, then you will need to make sure that they can handle another season. If you they are all-season tires, you probably get a set on the new car, so you might not need the extra set for quite a while.