How heavy am I?

Truck: Either the Kenworth W900 with 4 axles or the Peterbilt 389 with 4 axles.

DLC: Heavy Cargo Pack DLC

Firstly, choose the Transformer as your cargo. (As it is the heaviest cargo available in the game.) (The length and the destination of the job doesn't matter.)

Secondly, attach the cargo and go to a gas station and fuel up. (Just to make sure it's heavy enough, although some people said that fuel doesn't affect the weight of the vehicle.)

Thirdly, just drive to any scale around the map (Some gas station have them too, so it would be better to find one that have it and fuel up there.) and scale up.

Finally the achievement should be yours.