My last F250 (99) had 33/12.50/16s (stock suspension) on it when I got it. They worked fine. My current F250 (04) had 35/12.50/16s on it when I bought it, but it has a leveling kit installed. They worked great too, but would rub slightly if I turned the wheels to full lock. I would say that you can go to 33s with no leveling kit and 35s if you bring the front end up a couple of inches to level out the truck.