New guy old subject..tires

Something I guess I never thought about speed rating. Driving to colorado and back this year I cruised for the most part 65-70 mph which puts trailer tires at their max speed. So at 65 are st tires on the verge of destructing, or is that just a speed in which that's all they certify them for. If I cruise 68 am I looking for trouble? Answering other questions on rim size and wheel well I have one of those trailers that sits kind of low around the tires instead of up in the air. So I'm guessing from the looks of things I might and I that's a big might, may be able to fit a 225 75 15 at to the absolute most. I have 1 1/2" between the inner tire and wheel well and about 4" around the top.

Edit: after doing a little research I found on goodyears website stating that if st rated tires are going to be towed between the range of 66-75mph that you should add 10 psi above the max cold pressure and also this does not increase load capacity.