Wow...Let me get this straight. These world wide tire manufacturing companies such as Goodyear are stamping a speed rating on the tires they sell as a marketing ploy to sell more tires when they know they are not rated for that speed? I do not believe they would do that anymore than they would stamp a weight rating that was incorrect. Please post some references for the information you are quoting. I would like to do some research of my own.
The load equation for ST tires has not changed since the 1960's. At that time the higher load capacity for ST tires vs LT tires was justified based on ST's have a 65 mph max speed while LT tires could run faster but with lighter load.
I suggest you look at the load/inflation tables for identical sized St and LT tires and you will see the ST tires are rated for +10% to +20% more load capacity. Now when ST tires were facing stiff import duties in 2016 one argument made was that ST tires were really more like passenger tires than commercial grade tires. It was pointed out that they did not have a "Service Description" (Speed symbol and Load Index number). So the tire companies simply added a Service Description.
I have asked, more than once, what new tire technology is being used in ST tires with their new speed rating, some above 100 mph but no requirement for increased inflation or reduced load. So far no one has been able to identify the new "magic" components that overnight gave ST tires the big improvement. The only explanation I have been given for the improved load capacity of ST tires is that they are not subjected to torque loading as they are on trailers with no driven wheels. This might make some sense but they the next question is why don't tires in trailer application on heavy trucks get rated for higher load capacity? No answer is yet forthcoming.
If in fact it is possible to deliver load & speed durability that is greater than LT tires one would think this improvement would have been applied to LT and maybe even P type tires but no such improvement has been made by any tire company.
Tire load formulas are basically Air Volume x Inflation Pressure = Load. There are some factors applied based on expected service (percentage of time the tire is expected to be fully loaded)
RE the 30 minutes. This comes from the SAE speed test which is a 30 min step speed test.
Load formulas and 65 mph speed associated with ST tires come from
Tire & Rim Association.
SAE Speed test for passenger tires is
J1561 and for LT tires is
J1633.
Here is some info on speed testing in general.
