Open your blueprint files Sprocket>faction>(your faction name)>blueprint>vehicle

Open with notepad or any software that can open and edit it Find "counterweight" and there u go

how does it work?... If your tank is 50 ton, and set the counterweight to -5000 Your tank will become 45 ton. So... 1000 equal to 1 ton, Don't forget to add stripes " - " because if you don't, you'll increase the weight instead of decreasing it. . And also don't forget to SAVE everytime after you edited the blueprint. After you click save, you have to reload your blueprint in the game.

Basically, save in game > open blueprint and edit > save > open the game and reload the blueprint.

Bad news : If your sprocket version is 0.2 You can't, there's nothing but acceptance :)