There are two types of cargo trailer tires: bias ply and radial.  Bias ply tires are often used on economy trailers and utility trailers.  These tires offer a lower purchase price while sacrificing long term wear and performance.  A very popular upgrade option are radial tires.  Advantages of radial tires include better steering characteristics and less rolling resistance, which increases gas mileage.

The Radial design prevents the bottom of the tire (where the rubber meets the road) from flexing, unlike the biasply tire, which does deform under load. 

Additional benefits of a radial tire vs bias ply are:

1) Softer and quieter ride

2) Improved fuel economy

3) Flatter, wider footprint for longer tire life

4) Runs cooler than a bias ply, minimizing risk of a blowout on the highway

5) Better tracking

You can see a wide variety of trailers for sale at www.TrailersPlus.com