Am I missing something? I started the game in 1 player mode and it took me to the tank creation page, but I cannot find any combination that leaves the weight exactly between 25 and 35 tones. If I use the small chasis and use the heaviest things, the weight is under 25, and if I use the medium chasis and remove all extra things the weight is above 35.
EDIT: Oh, I reloaded the game and it seems the target weight is random. I will check now.