Some safety advice,

Never use passenger car tires on a trailer. The sidewalls aren't made to withstand the abuse that's given by your trailer especially with 3,000 pounds sitting on top of them bouncing up and down at 75 mph. Notice the shredded blown out trailers sitting on the side of the road. You get what you pay for. If you want to know your boat weight just e-mail the company or pull it up from their web site. Either way I'm sure the info is easily obtained.