So I'm looking for some truck tires for my brother. Now he lives up in the mountains and is able to get a certain tire at Walmart that is actually really good. The size that he told me he got wsa 31x10.5r15. Now his problem is that they only had two in stock and the people keep slacking about getting two more to the store. When I call any of the Walmarts down here in the front range, no one knows what I'm talking about. Now if I change the size I'm looking for to a 215x75r15 everyone can find the tire. Here's the tire, http://i.walmart.com/i/if/hmp/fusion/06_Q3_TireFlyer.pdf#search=" supersport tires walmart" It's down at the very bottom of the ad. Now my question is what's the difference in size standards? I've never had a truck so I've never had to order truck tires. Should these be a similar size tire that will work with the other two that he has? Thanks for any help.