For 31 x 10.5 R15 AT tires To protect the spare wheel/rim set from dust and sun. The material is completely opaque so I believe it protects the tires even in bright sunshine. I didn't actually test the water resistance but she spilled a cup of water when I poured it over her. Best of all, the bag is actually slightly larger so it easily slides over the hoops, and it zips at the bottom. to close and rope to tighten it before flapping