Varies by state. Here in TX, it's 85 in the country on freeways (I think you call them "motorways"), 75 when they pass through cities and typically 45-55 in urban areas on all other roads, 30 in dense residential areas.
And we don't call tollways freeways. If you have to pay a toll, it's a tollway. If you don't and the speed limit is high enough, it's a freeway. The ones who actually USE tollways are the ones who think that the fact that you have to pay to use them means that there are less people on them and they're therefore faster. Unfortunately for them, a lot of people similarly ignorant of how economics work use the same logic, meaning that you're usually better off using other roads.
It's complicated, and it takes quite a bit of getting used to