When I bought new tires last year I researched the heck outta them. I ended up with Carlisle Radial Trail T/A's. They were speed rated to 65 mph. What that meant, at least on that specific tire, is that they are rated to hold their maximum load rating at maximum inflation at maximum speed of 65 mph.
I forget what the exact percentage was, but for each mph OVER the maximum speed limit, the weight rating dropped by some percentage.
Therefore the maximum speed rating doesn't mean that the tire won't perform above 65 mph, it means that the load rating dropped off over 65 mph.
HEAT is what kills tires. You can do 55 mph and not have enough air pressure and get significant flex in your tires and your tires will heat higher and they will fail faster. You can do 65 mph and have sufficient air pressure and minimal flex and your tires won't generate as much heat and will last longer.
Can you do 70 mph with properly inflated tires that are rated higher than you need them and not have issues...? Probably, but I doubt too many tire manufacturers will come out and put that in writing.
