Ever large simi truck in rear life will have a 2 speed rear which doubles the amount of gears the transmission has I think this could be brought into the game and the button you would use would be square on ps4 and x on xbox idk on pc but it would make the game more realistic, enjoyable and immersive.
Mudrunner 1 was originally on military Russian trucks trucks though. With Mudrunner 2 seeming to be have a American semi focus from the get go however, they might figure that out.
However it depends how the physics go. In Mudrunner 1 you can barely handle a semi in 5th gear as it, and faster scout vehicles like the Blazer are basically undrivable in top gear. So if Mudrunner 2 has the same physics, what's even the point of adding a high range gearbox? It'd never be used anyway.
So yea, if we're to have low and high gearbox then we need the game physics to be done over.