I wouldn't measure to the edge of the rim, it can be out of round. Measure to the axle housing right where the backing plate bolts up. Then pop the grease cap off the hub and measure spindle to spindle. As far as tires go, at the trailer dealership I worked for, the smaller trailers would come with the ST tires and the big heavy trailers would come with LT tires. I think the biggest tire I saw was the Goodyear G159. When customers wanted to buy new tires we would recommend LT tires with the correct load range (normally 'D' or better). They are cheaper than the ST tires. Even after I moved on, when I bought tires for my trailer, the salesman would try and sell me ST tires, but would eventually concede that the LT's would work just as well. Now this is just my opinion!!! It only matters to me!! John