What you wrote here is patently and demonstrably false. If you bother to read the Constitution, you would understand that there can be no Federal law setting speed limits. The infamous 55 'set' temporarily by Nixon and 'permanently' by Carter were not Federal speed limits. They were Federal coercion of states to lower state limits or face reduced federal funding.
Not all states have the same maximum, nor minimum speed. This makes a lot of sense, because the tiny eastern states don't gain much value in higher speed limits. Out here, higher limits allow us to spend less of our lives on the highway.