How to calculate a mesh's mass using physical materials

Hello everybody!

I’m having trouble wrapping my head around how the mass calculation is being made.
I’m trying to get a vehicle to have correct mass distibution, but the mass calculation that the engine makes is completely wrong. If I’m correct it should be around 1500 kg, but if I attach a physical materials to the shader mats I created the mass goes down to 18 kg.

The mesh has multiple materials, I created physical materials for every one of them, attached them, but when I change values, only the first sub-object’s material makes a difference.

For example: I change the density on the chassis, the mass changes (it’s not correct, because I basically calculate a thin sheet of aluminium). But when I change the engine’s density, the mass doesn’t change.

I would be grateful if you could help me.