Is that with a load? 50 on the tire is the max safe level with a max tire load. 40 is plenty when the trailer is empty. I used to build 2 place trailers for a company. Personally, i check my trailer tires whenever i put a load on them. Otherwise use good eye judgement on how much the tire is squatting and adjust accordingly. For a smoother ride, let some air out when it's empty and it won't bounce over every little bump.