This transmission pack for Volvo includes 6, 4, 16 gearbox. Use 14 or 16 gearbox if you are in a map with hills.
See these screenshots for details :D.
Transmission Mod for Volvo screenshots:
How to install Transmission Mod for Volvo 2012 ETS2:
- Download this mod.
- Unzip it!
- Paste the .scs into the My Documents/Euro Truck Simulator 2/mod folder.