Are you going to use these on the street or just trail? Radial or bias tires? I've seen what happens to 12.5 on an 8" wheel but the guy was running full tire pressure. It wears out the center. Like Mini said, the air pressure will makes the difference. On the trail you'll be good but on the street you will have to run a lower tire pressure to get the tread to wear more even. You will show a decrease in fuel mileage due to the more rolling resistance but if fuel mileage doesn't matter then run them on a 8" wheel.
