I was going to put a dump insert in my truck. Gvw 8600 title says truck weighs 4300 lbs so that leaves me 4300lbs for insert and material. But i get registration in mail today and it says the weight is 5340 lbs. What gives? What numbers do i use? How do I figure what my towing capacity is? I have a gvw on sticker and front and rear axle gcvw( might be in wrong order).