You could go as small as a 265/75R-22.5 and still have sufficient tire to take the rated 10,000lb load rating of the tag axle. As to tire size, the only reason that 12R's were originally used, was they were the only tire that could take that kind of loading-realizing that tires when the bus was built were still nylon bias ply tires. Now with the excellent steel radials, you can go smaller and not have to worry about loading. At 13,000lb you could use either a 11R or 295/75R-22.5 16 ply for the front. For the rear at 22,000lb, a normal 11R or 295/75R-22.5 at 14 ply would be enough. Most converters will not come close to the gvw rating for passenger carrying capacity. For instance, my bus is rated at 13,000lbs front and 23,000lbs rear. I went to 11R-24.5's that are 16 ply since they will take that amount. At full pressure, the 16 ply tires will take 14,000lbs front and 26,300lbs rear. With only 10,500lbs front and 20,500lbs rear, I run 90psi all around, and that is even a bit high. Good Luck, TomC
