Trailer Tires

Sounds like your open trailer has 3500lb axels. Typically, the higher load range tires have a higher speed rating too. For my 3500lb axles I use the load range D tires and iirc they have a speed rating of 82mph compared to the 6 ply load range c tires that have a 68mph rating. In other words going the better tire gives you alot more and is worth the upgrade in my opinion. A really hot day at 80mph on the highway those load range D tires will hold up alot better as speed and weight play with tire durability. As far as the nitrogen thing I agree with most comments. The right tire pressure means more than what you use to fill the tire besides you should check your tires regularly anyhow.

Also I feel like this is why the 3500lb axels on enclosed trailers get a bad rep for blowouts! Those cheap load range c tires aren't made to carry a 6500 lb load all the time and especially at highway speeds.