We all use threaded tires. We use tires for 8 inch rims, or 6 inch rims depending on the organization. If you want to keep the rear rear 8 inch rims, you can use 15x6.50-8s, 16x6.50-8s, or 16x7.50-8s. If you are willing to get some rear kart rims, you can use 15x6.00-6s, 13x6.50-6s, or 13x5.00-6s