I don’t really know myself, I’m slightly confused. Weight is useful because it amplifies the crash in PVP and it kind of gives that extra attack power. However, I’m not sure whether weight affects speed (slowing the car down, etc). There’s not much I can say.
EDIT: Panwellz himself said in another topic that weight is actually good, as it does help damage other vehicles more efficiently, which is what makes big vehicles powerful.