Rules in NTPA tractor pulling are assigned based on tractor class: most rules govern permissible tractor weights, but also cover engine size and power and vehicle modifications. Most restrictions in the NTPA fall along the basis of tractor weight, which accounts for the many different classes of tractor pulling. Assuming entrants have vehicles within the permissible weight limit, the primary rules are to pull tractors with as much weight as possible as far as they can over an official track, which usually spans 30 feet wide by 320 feet long.
In the class of modified tractors, all equipment must have a 20 inch draw bar and a tire size of 30.5 x 32. Vehicles in the Modified RN class have a weight limit of 7,500 pounds. They must have turbine engines recognized and approved by the NTPA, and cannot have a horsepower over 5050.
The next class is Light Unlimited, which is indicated by the initials GN. Rules in this class include no turbine engines and a maximum weight of 6,000 pounds.
The Modified GN class has a weight limit of 7,500 pounds. Vehicles may have three or four blown automotive engines, two aircraft or two marine engines or a 12-cylinder industrial engine. Regardless of engine type, they cannot have a horsepower exceeding 5,800. Lastly, the Unlimited category imposes a weight restriction of 8,000 pounds.
In addition to tractors, the NTPA classifies 4-wheel drive vehicles and stock trailers. These machines are also subject to engine modifications, weight limits and other impositions.