The tractors are divided into different weight classes based on the tractor weight, the weight classes starting at 2500 and ending at 14000 pounds. The tractors could go in any class they choose, a lot of tractors will weight their tractors up, so they fit into the higher weighted classes. In order to be able to compete in Antique tractor pull competitions your tractor must have the original speed the tractor is supposed to have. Some pulling competitions will have a ‘dyno’ so you can ‘dyno’ your tractor, which means hooking up the PTO shaft on your tractor to the machine to test the horsepower. If your tractor shows it has more horse power than the original tractor is supposed to have, you get put into an alternate class called ‘Modified Tractors’. They do this to avoid the tractors that have been modified always winning, they want a fair competition.