Although implement tires would be best, I'd suggest with the truck tire route if it's just a typical old hay rack. We have a scrapyard around the corner where anyone's free to dig through their tire bin and buy them for $10 ea. Almost all my wagons/trailers have tires from them, many of which are getting to be 20 years old, and thus far no problems. All of them have tubes in them. Most of our fields are a few miles down the road too, so they get relied on for road use. None of the hay wagons that have those tires hold more than about 160 bales at most - typically 16' racks, 8' wide. I also have used truck tires on the fronts of a couple of my older and smaller tractors. The only wagon that doesn't have used truck tires on it is the larger 24' metal round-bale wagon, which has larger/wider/heavier tires on it, and I wouldn't want anything lighter-duty for it.