Even with these console commands, hitting 400 km/h on a straight highway usually results in the truck taking flight like an airplane. The new terrain collision boxes in the 1.55 update treat a landing truck like a bomb.

Deactivating the native "Truck Speed Limiter" within the game’s engine mechanics.

If you want to troubleshoot a specific speed mod, let me know: What are you currently running? Which truck model are you trying to upgrade?