A few thoughts to respond to some of the comments on this thread...
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ST tires have had speed ratings for decades; IIRC it was the early 90s when the current system of ratings was implemented. It seems that a speed rating "L" is very common, which as others mentioned is 75mph.
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Fuel mileage and speeds? Here are my experiences from a long trip a few weeks ago; I wanted to gather some data on this particular topic, so all data is based on a single fill up, hand filled to the very top each time:
-lowest mileage on trip, 7.4mpg. Conditions were little wind, but lots of curves, lots of elevation changes; speeds averaged ~60-65mph (kept with traffic on 2 lane highway)
-highest mileage on trip, 10.1mpg. Conditions were calm, straight interstate, little elevation changes; speeds averaged 70mph to keep up with most of the trucks
-second worst mileage (8.1mpg) was ~65mph and facing a quartering headwind. Straight headwinds seem to matter less.
-other mileages were from about 8.5 to 9.5mpg. Speeds ranged mostly from 65 to 70mph.
Learnings?
-driving 60mph vs 65mph vs 70mph did not prove to impact mileage significantly. As above, the best mileage of the trip was at 70mph and the worst was at 60-65mph.
-conditions such as winds (especially side winds), and/or sudden or significant elevation changes and/or curvy roads had FAR more impact on mileage than did changes in speeds, as above.
Note that this is the observations for my rig, and YMMV - pun intended
My rig: 2020 RAM 3500 Cummins HO with Aisin, and 2021 Montana 3790RD, loaded to max as we live in it about 6 months per year.
A starting point for your rig; for a diesel engine, the peak torque RPM is often the sweet spot for mileage. On my truck peak torque is ~1700rpm and I think that is ~68mph. When I run at 60mph IIRC my RPM is at ~1450-1500 and the truck will sound like it labors on inclines, then may want to downshift earlier. I try to stay at 62-65mph up to 70mph on interstates, which seems to be the sweetspot.
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Tire temperatures and speeds?
(my experience includes over 1m miles towing, commercial for years)... People can get all caught up with looking at their TPMS readings and temperature readings while driving, but note that tire manufacturers do not specify max "hot" tire pressures, nor do they specify max operating temperature limits.
So, what are the keys to preventing a tire blowout?
First and foremost - PROPER TIRE PRESSURE!!! If you have an L rated tire (like most new Montana's) and you fill it to the proper tire pressure with proper weight loading, your tire will stay within the designed limits for pressures and operating temperatures. Say it another way - driving at 60mph vs 70mph will not likely be the cause of a blowout if you maintain proper loading and proper tire pressure (of course assumes tire is in good condition)
Second, if you have a TPMS, some systems measure tire temperatures as well as pressure (often will warn at 156F). I think the biggest value in a TPMS system is not to stress overmuch on the actual pressures and temperatures, but to monitor for any sudden changes. So, if all of your tires start at 110psi and a couple hours in they are all at 130psi, should you stop and let air out? Of course not; we all know the values are to be measured cold. But if all tires are at 130psi and suddenly one goes down to 110psi, you should suspect a leak and get over to deal with it.
The same with temperature; should you stress if you see tire temperatures of 130 or 140F, but all tires are similar and none changed suddenly? Again, no worries. However, if all tires are at 130F then suddenly one tire goes up to 150F and climbing, that's when you know you need to get over asap to prevent a problem (could be stuck brake, or dry bearing, etc).
My tire experience so far? I now have over 20,000 miles on the truck and at last service it was scheduled for tire rotation; all of the tires were identical in measurements so they didn't even rotate them (14/32 tread depth, 18 new). And, with over 9,000 miles on the trailer tires, they are wearing very evenly and look almost new.
I do hope the above data is helpful to others!
Brad