If you're looking to fit the largest tire size possible on a 20x12 wheel, you'll be limited by a few different factors. First, you'll need to make sure that the wheel has enough backspace to accommodate the tire. Too much backspace and the tire will protrude too far out from under the fender, while too little backspace can cause clearance issues with suspension components or brake system parts. Secondly, you'll need to make sure that the width of the tire doesn't exceed the width of the wheel lip. A wider tire than wheel lip can cause rubbing issues when turning or during suspension articulation. Finally, you'll need to take into account the height (or aspect ratio) of the tire. A taller tire will have a larger diameter and can cause clearance issues with bodywork or other obstacles on the road. Assuming that your 20x12 wheel has adequate backspace and width for your desired tires, the largest possible tires would be 315/35R20s. These tires are about 11 inches wide and 35 inches in diameter.