I only drive Scaina or Volvo all it's because i have dlc's for them only in the game :3
When i drive big & heavy cargos i use 14 gear and if the cargo is small & less heavy i use 6 gear.
I love old scaina series much more because they have clear vision and analog details on the road computer. It makes much more feel realistic anyways.
Currently my favorite truck is Volvo FH7, it works on every type of cargos, fast and stable.