The common sizes for heavy trucks are

11-22.5 and 12-24.5
Unless they are running super singles they have one of the two.
Thats where all the reasonable prices are, and availability too.

I think this would be a good request for the Virtual Coach thread. It would certianly save a bunch of cash.

I think I remember seeing busses for sale on the internet with these "truck" sizes. Couldnt tell ya if they were switched or older busses used common sizes with trucks?