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:

 settyrescale(number) - Determine tire size. (i.e., 4x4>settyrescale(0.5) will give you tiny tires.)
 hanyman()            - Repair truck
 ironman(1)           - No damage to truck
 freeman()            - Unlock all modes