SpinTires and MudRunner:
SpinTires and MudRunner have a system where a button is pressed and it engages the AWD system. AWD helps give the vehicle more traction and control by sending power to all wheels. The downside of this however is that fuel consumption goes up and is useless on surfaces that provide lots of traction such as Asphalt. Some Trucks in Spintires like the D-535 have AWD on all the time. Some vehicles, such as the B-130, have no AWD and thus will struggle when crossing tough terrain. Such vehicles are not designed for heavy offroad usage and are best driven only on paved roads or other hard surfaces.
SnowRunner introduces the system of being able to buy AWD upgrades for trucks that do not have AWD. Certain trucks like the Ford CLT9000 do not possess AWD at all, trucks like the Fleetstar F2070A or White Western Star 4969 have their AWD hidden away in an upgrade car and can equip it once unlocked, and some trucks like the Voron AE-4380 or the Tayga 6436 have theirs always on by default. Some trucks have engagable AWD from the moment it is purchased like the International Paystar 5070. AWD is engaged the same as in the two previous titles by hitting a button though depending on the control scheme, the clutch may need to be engaged to do that.