T-series fuel tank question

I was looking at the fuel tanks of the Gavril T-Series, and I saw that both fuel tanks carry ~450 liters of diesel. But when I started the game loaded the T-Series, I was shocked to see that the truck only has ~450 liters of diesel instead of ~900 liters of diesel. (450 + 450 = 900) Does this mean that the game does not calculate stuff like this? I am just wondering why.

Reply's very welcome.

 

I think only one fuel tank actually does anything, like the engine in the belaz.

 

Uhmmm, what a shame actually.