R.I. Gen. Laws § 31-6-2: Computation of gross weight.

For the purpose of computing fees based on gross weight, the gross weight shall, in the case of motor trucks, tractors, trailers, or semi-trailers, be the weight of the vehicle in pounds plus the weight of the maximum load to be carried. In the case of passenger motor vehicles, the gross weight shall be the weight of the vehicle plus the sum of the adult seating capacity multiplied by one hundred fifty pounds (150 lbs.).