Even got to an engineer at Tire & Rim Association and based on his answers it was obvious TRA had not thought about ST type tires or the unique nature of most RV trailer applications for many years so I believe he acted a little surprised when I identified the engineering analysis that indicated that the current loads too high for many trailers and that the load capacity should be decreased by 15% to 25% or more if the failure rate was to be decreased to a more tolerable rate rather than the 5 or 10% or higher some are reporting. This is not his or TRA fault as I understand it is the responsibility of tire company representatives to TRA to present and review technical matters.
Anyway here is the bottom line.
For Goodyear Marathon tires only:
There is a Goodyear Tech bulletin PSB#2011-13 that uses information from TRA.
The tech bulletin says that the normal max speed for Marathon ST type tires is 65 mph. This speed can be increased up to 75mph only if the inflation pressure is increased by 10 psi. This new inflation can be 10 psi higher than the pressure molded on the sidewall of the tire that is associated with the tire max load.
The load does not increase beyond the load molded on the tire sidewall.
Goodyear does not support or condone operation above 75 at any time.
The trailer owner is responsible to confirm the rim is rated for the new inflation level.
While TRA indicates a further increase in speed might be achieved Goodyear has decided not to adopt this option.
The person I talked with at Tire Rack said they had already changed their web site to indicate this applied to Goodyear tires only and that they were attempting to learn if any other ST tire manufacturers were willing to make a similar change. As of Saturday Oct 19, 2013 they had not identified any other tire manufacturer willing to make an increase in max speed above 65.
Info added 10/26/13 10:51 AM EDT
If you have a GY Marathon ST225/75R15 LR-D it is rated for 2540# @ 65psi and a max speed of 65 mph. If you increase the cold inflation to 75psi the tire would now be rated for 2540# @ 75psi and a max speed of 75 mph.
The example currently only is OK for Goodyear Marathon ST tires as I have not seen a published document from any other tire manufacturer.
The pressure increase must not exceed the max pressure capability of the wheel. You need to find a max pressure rating stamped on the wheel or get something in writing from the wheel MANUFACTURER.
This Max speed is like your engine red-line. While it might be possible to exceed this speed for short periods it will significantly reduce long tern durability.
I have sent a copy of this post to both TRA and Goodyear. If they ask for corrections or additions I will edit this post accordingly.