Not sure where you get the "Load Rang D specs are more like ... 80 mph." By the way you said it, you seem to think any LRD is rated for 80 mph, and that's not true. Although there may be some specific tires which are rated for 80 mph, ST tires only need to handle 65 mph when using the normal load/inflation specs, and that's all many are rated for.
For all ST tires, you are allowed to go to 75 mph by increasing the pressure by 10 psi over that given in the tables (not to exceed the max pressure), and to 85 mph by both increasing the pressure by 10 psi and derating the load limit by 10%.
