I measured them on a balance scale made with hinges.

How did you find the mass of all of these? old radars? (which can detect mass with simple math)

jacobssgt2,
Few guesses:
- Wheels, tank wheels, train wheels for lowering the vehicle CoM in order to reduce the risk of derailment.
- Propellers to keep some momentum under acceleration/deceleration.
- 2x weight for fluids in interiors/custom tanks to make ships sink easier because weight difference between components are low.
- Sub-bodies are stronger when they are heavy.
- Light weight fuel, character, ammo...etc eliminates high risk of CoM shift in vehicles during cruise which can cause accidents.
- Extra mass for antennas, ropes allow them to react under physics.

Why do devs hide so many mechanics from players?

And also, i did a small calculating mistake, i calculated for 0.1 per L instead of 0.2, so the liquid tank should be even heavier, it's weird

The tanker holds 280kL, and yea custom tank, and i measured it's heavier by your table up there, + the weight of the coal hopper themselves (500 for large 250 for med), but the doubling in speed is quite surprising, considering it's quite a heavy truck to begin with

It may be in relation with drag. Similar to adding weight to jets which increases speed.

Coal truck goes 130kph, while the tanker truck (both have the same drive train and base, the only difference being the Tank/Bed) Tanker does 260kph

10800 is coal in ducts/hoppers in your truck. That makes 10800 mass for the coal.

Does the tanker hold fluid? I presume it is custom tank. That makes 56000 mass for the fluid.

How did you measure that coal in the truck is heavier than the fluid in the tanker?

i've been told*