Interesting idea and worth considering Bill. But I thought there was some reason for using ONLY ST tires on a trailer. Did you ever hear that theory? I can't recall the reason.
You're right, they do recommend ST tires
for trailers. (S)pecial (T)railer. They have stiffer side walls to help provide more lateral stability and they are usually load range C or D, but the ST205/75R14 which is the common 14" ST tire is too tall to fit Trilliums unless you change the axle
height. I think the lower profile (P)assenger tires
with a wider tread can have a high enough load rating and the shorter sidewall height should provide good lateral stability, and fit the Trillium
. The original A78-13 tires on these Trillium
trailers(I just checked the spare on my old 4500) had a load rating of 1060 lb., and I think the 14" (P) tires shown were around 1400 lb. The ST175/80D13(D=Bias ply) tires on one of my trailers are rated for 1360 lb. and replace B78-13 tires. I hope this helps.