The gearbox can be accessed by left clicking on the round white shift knob, or pressing the "R" key (and moving the mouse). Then selecting the shifter position by moving the mouse.
1. Gearbox automatic
This position makes the automatic transmission. Has possible gears 1,2,3,4,5 & R depending on your current truck. In Hardcore mode, Diff Lock is disabled in this position. Useful for asphalt road, going downhill and precise positioning of your truck for parking or attaching trailers.
2. Gearbox 1
This is a manual transmission ﬁrst gear, low range gears. Your typical workhorse, this gear and force acceleration gets you to places efficiently. The closer stick is to terminal position, the faster truck moves. In Hardcore Mode allows usage of Lock Differential function.
3. Gearbox 1+
This position forces engine’s high-gear – when you need to go faster, good for river crossings. The engine will stall if you use it to start or to climb up steep hills. Stalling can be prevented by hitting brake when RPM arrow is still in red zone. When going downhill you can use Park Brake in the beginning, switch your gear to 1+, release Park Brake and easily start moving in this gear.
4. Reverse -1
Gearbox for going backwards. Allows usage of Lock Differential function.
If you right click when gear selector is up, you can vary the speed of the truck by adjusting the position of the stick in the gear branch. Implement it on steep hills to control your ascend speed, or to control your fuel consumption on the muddy parts. To use Force Acceleration with XBOX360 controller press Right Stick, then Left Stick.
Notice that when you right click the leaver, the knob's upper circle will turn from grey to white, so that you know that force acceleration has been activated.