One 'hull space' is "50 tons" because it displaces 50 tons of water. . .  or rather, that was the aid-to-imagination we all adopted years ago to better picture our ships.  It doesn't have any game effect other than the 1:50 ratio.

No, did not mean the HullSize-tonnage ratio (and also 315,4t * 50 are not 4.000t but rather = 15.770t of displacement).

The only logical conclusion is what Eric wrote, that the other tonnage is used by the not accounted standard materials, because what determines the speed of the vessel is the 4.000t tonnage and not the used 315,4t of TN materials used to construct it, because "empty space" within a spaceship does not slow a spaceship down like it would inside an atmosphere. Size does not matter in a vaccuum to accelerate an object.

I was doing some calculations to try to imagine how big those TN ships are. So I took the 315,4t as the real used materials and the 4.000t as a volume size - it would fit nicely for example to the actual US Space Shuttle. That wheighs roughly 10kg per 1m³ and the example ship would wheigh 15,7kg per 1m³. Assuming these spaceships have a similar ratio to wheight and material use by standard sea vessels, this example ship would have a real volume of roughly 20.000 m³, approx 78x21x12m in cubic size (assuming a ship lenth-width-depth ratio). Quite reasonable for a 55 man crew ship.
Only problem is that the speed calculations would not fit real physics - but oh well - the numbers do work inside Aurora... and this is just for the imagination 

One 'hull space' is "50 tons" because it displaces 50 tons of water. . .  or rather, that was the aid-to-imagination we all adopted years ago to better picture our ships.  It doesn't have any game effect other than the 1:50 ratio.