I mean it really depends on what they're geared out at and how much power they have and what they're hauling. Theoretically you can do 150mph in a semi pulling max gross if all those factors line up right and you don't care about your life or the lives of others let alone the fuel bill. Typically an ungoverned truck is geared out to do about 93mph to 118mph depending on the transmission and gearing, but it's incredibly unwise to run those sorts of speeds for very long because I believe the fastest load rated semi tires you can get top out at N rating which is 87mph if I recall correctly or thereabouts.

And like yeah there's a safety factor built in, it's not like the tire instantly flies apart if you exceed the speed rating one time, but now you're getting into factors like how hot is it outside and how hot is the pavement and how much weight is on that tire and how old is it and what's the exact psi and how long are you running that fast. I could see a bullhauler hitting triple digits at night when loaded but they're likely not holding that speed, they're probably cruising at about 90ish for extended periods of time and slowing down periodically before they get to curves or turns which will let the tires cool off a bit.

They can also get ticketed for going that fast despite what everyone says. Even if they are loaded, but they had better have been remaking Smokey and the Bandit and going full dumbass for that to happen. More likely the cops just wait until they're not loaded or send their company the footage and the ticket(s) as a warning to chill out.

Realistically speaking though - if you know what you're doing and have a lot of experience and you're in really well maintained equipment on fast stretches of highway, you can safely do 90mph for extended stretches of time even in a loaded semi. At night. In clear weather, and light or no traffic. Which is the actual reason they don't instantly get a shitload of tickets while beeves are falling over dead in the trailer stopped on the side of the highway.