I came across a fact sheet from Discount Tire Co. about trailer tires, that I find interesting. I copied part of it below:
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Speed
All "ST" tires have a maximum speed rating of 65 mph.
As heat builds up, the tire's structure starts to disintegrate and weaken.
The load carrying capacity gradually decreases as the heat and stresses generated by higher speed increases.
Time
Time and the elements weaken a trailer tire.
In approximately three years, roughly one-third of the tire's strength is gone.
Three to five years is the projected life of a normal trailer tire.
It is suggested that trailer tires be replaced after three to four years of service regardless of tread depth or tire appearance.
Mileage
Trailer tires are not designed to wear out.
The life of a trailer tire is limited by time and duty cycles.
The mileage expectation of a trailer tire is 5,000 to 12,000 miles.
Why Use An "ST" Tire
"ST" tires feature materials and construction to meet the higher load requirements and demands of trailering.
The polyester cords are bigger than they would be for a comparable "P" or "LT" tire.
The steel cords have a larger diameter and greater tensile strength to meet the additional load requirements.
"ST" tire rubber compounds contain more chemicals to resist weather and ozone cracking.
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If, as they say, a trailer tire loses 1/3 of its life by its 3rd birthday, no wonder we have so many blowouts.
As for why to use a trailer tire, it is almost solely because they have higher load capacity. Unless "demands of trailering" means something; I don't know what those demands would be other than carrying the load. They are supposed to resist weathering better than other tires, yet they still weaken dramatically in 3 years, so how great is the alleged weather/ozone resistance?
