In gameplay when you are in a truck: Press q, i , f12 at the same time, then press the ~ key. This will bring up a console in the lower left of the screen with a heading like this: 4x4> Press enter after each command is put in; the commands are as follows:

Code - Result

settyrescale(number) - this will determine tire size.
hanyman() - will repair your truck
ironman(1) - no damage to truck
freeman() - will unlock all modes