Note: You may want to seek your administration's blessing before soliciting such a donation, but this "gift" wasn't a gratuity in nature and most business owners are eager to help their public servants with no expectation of special treatment.
Look for a tire in the range of 150% to 200% of your body weight. The tire I'm working with these days is around 427 pounds, but I weigh 240 and am hoisting only a portion of the tire's mass at any given time in the movement.
Here's the technique: Squat deeply, hands inside the feet, forearms flat against the tread, maintain a neutral lumbar, and let your legs do the work. When you get beyond 45 degrees, the tire will tend to right itself; release your underhand grip and forcefully check the tire away with open palms before chasing it for another rep. Work dynamically. If it's too difficult to do a continuous set of 10‒12 repetitions, get a lighter tire rather than reducing reps.
For added challenge between sets, beat the tire with alternating overhand sledgehammer strikes. I signed one out of our property room for this purpose. Again, this requires no cost but reaps huge dividends. This movement works the core and shoulders while keeping your heart rate up between sets, and it activates the same muscle group that's involved if throwing overhand hard-empty-hand control techniques when top-mounting an actively aggressive suspect.
Don't be discouraged early on; the results will come. The first time I trained this way, I could muster only 25‒30 reps before exhaustion. Nowadays, I regularly do upward of 130 repetitions on tire day.