Drill a bunch of drainage holes in the bottom of the tire, at least one hole every three inches. This is to keep water from pooling when it rains. Incidentally, used tires combined with water make the perfect breeding ground for mosquitoes (more on the tragic global consequences caused by mosquitoes incubating in tires here). With a bunch of drainage holes you won’t have a problem.
Got more used tires? See our used tire composter here.