This is conversion of FS17 script. This is not a mod. You need to put TerraTrac.lua to your mod and configure it in your modDesc.xml and vehicle.xml. Everythnig is the same as in FS17.

copy this to your modDesc.xml:

<specializations>
	<specialization name="terraTrac" className="TerraTrac" filename="Scripts/TerraTrac.lua" />
</specializations>
<vehicleTypes>
    <type name="yourVehicleType" parent="baseDrivable" className="Vehicle" filename="$dataS/scripts/vehicles/Vehicle.lua">
		<specialization name="terraTrac" />
    </type>
</vehicleTypes>

copy this to your vehicle.xml (edit indexes to your i3d model):

<TracWheelsLeft index="0>15|3" node="0>15|0|1" animationClip="TracLeft1" sped="-7">
    <rotatingPart index="0>15|1" radius="1"/>
    <rotatingPart index="0>15|2" radius="1"/>
</TracWheelsLeft>	
<TracWheelsRight index="0>14|3" node="0>14|0|1" animationClip="TracRight1" sped="-7">
    <rotatingPart index="0>14|1" radius="1"/>
    <rotatingPart index="0>14|2" radius="1"/>
</TracWheelsRight>

If you find any errors or issues, let me know in comment or to kenny456@seznam.cz.