I take the basics for a car ,
spare water pump thermostat and thermostat housing gasket , and head gasket ,
radiator leak sealant and narrow nosed pliers ( helps pinch off the cut bars after a roo knocks the radiator against the fan , or if you go into deep water too fast and the water forced the radiator against the fan )
spare universal joint
spare wheel bearings , spare points plugs and leads , and dizzy cap .
spare fan belt and power steering belt .
spare water , 20 litres ( this is for emergencies , not for general use )
spare oil 4 liters minimum preferably 8
spare fuel and oil filters
2 liters moreys oil stabiliser
tube silastic gasket
can of spray degreaser .
WD40
Nulon "start ya bastard" ( no joke its what replaces areostart these days )
2 spare tyres
minimum 40 litres extra fuel , if I can I have a trailer with either a 120 litre spare tank on it or for longer trips , a 500 litre truck tank mounted on it , plumbed into the car fuel system thru a 3 way valve that allows me to swap between the car tank or the trailer tank . I got no idea if its legal but it works .
tool kit , I carry a few extra things , tyre leavers , patch kit , 3 claw puller , breaker bar socket sets , spanner sets ( x2 , I hate needing 2 of the same sized spanner and only having one ) screw drivers allan keys torx drivers and a bunch of one of tools like a spanner bent to get to the inside carbie mounting stud and things as this , multi meter electrician tape , wire cutters and fencing pliers , a couple different ball peen hammers . a sledge and a handfull of drifts chisels wad punches and a center punch .
I carry a kangaroo jack and snatch strap ( tow strap that flexes , OK ? ) and a come-a-long winch with a bunch of d shackles , and a tree trunk protector .
I carry a machete and hatchet for getting grass / shrubbery / whatever for extra traction in mud or bulldust holes
I also carry a butcher knife under the car seat , to make the best use of any animals we happen to hit on our travels .
and a multitool in the glovebox .
thats the basics , I customise it a bit depending on what vehicle we are taking and where we are going