A lot of what you can legally haul depends on your tires. It is federal law that you cannot exceed the carrying capacity of your tires. It will be stamped on the side of the tire. The carrier did not charge you an outrageous charge. I charge dumps at $90 per hour. I'm afraid you will be astounded when you get an insurance quote on liability insurance if it is classified as a "dump truck."
You will probably only be able to haul 3 or 4 yards legally. A C50 was not a very heavy truck. If the gvw is 16000 it is probably a 10000 rear and 6000 front and that's all they were built for.
Sorry if I implied their rates where outrageous - I know they got bills to pay too. And with the cost of diesel - those rates will most likely have to go up - if they have not already.
Maybe, with the cost of the truck, insurance and maintenance I'll lose money hauling the materials myself, but I'll still own the dump truck when its over to use on another property
I live by myself and having the ability to pickup smaller amounts of materials actually works better so I don't have so much to deal with at one time.
