From what I've seen, the tire carcass is much heavier on the trailer tires, more like a light truck tire in construction. I've even seen a few all steel construction trailer tires in much smaller sizes that a typical truck tire would be found in. There's differences in the tread, too. The trailer tire treads are really not very good for use as a drive tire, and even worse for a steer tire. Another thing I've notices, particularly with the tube type, is that they tend to be odd wheel size compared to a passenger or truck tire, 14.5 and 15.5 seem to be common. Around here, 14.5 is very common on mobile homes and lowboy trailers.
