Tonnage is a mechanic in Crossout.
Every cabin has a Tonnage and Mass Limit. Tonnage is the soft cap for a vehicle's mass, and is the maximum weight a vehicle can carry while maintaining optimal performance. The mass number in build mode will show if tonnage is within optimal range, as the number will be white. Should tonnage be exceeded, the number will turn yellow, along with harsh drops in acceleration and speed.
A vehicle's tonnage can be increased with movement parts.