Understanding the different aspects of tractor horsepower is the first step to comparing this selection criterion. There are three main types of horsepower that need to be assessed when making comparisons: gross horsepower, PTO horsepower, and drawbar horsepower, as indicated on Hobby Farms.
Gross horsepower is the total power output of the engine, including power lost by the engine by simply operating. PTO horsepower is the amount of power that remains after subtracting the power loss from the engine. It is the power that is left to operate the actual implements of the tractor, and is usually what is most commonly quoted by manufacturers when they give a value for the tractor's horsepower.
The last value, called the drawbar horsepower, is the power that the tractor exhibits when it is actually operating, doing tillage work, such as disking and plowing. Hobby Farms reports that, on average, PTO horsepower is around 75 percent to 85 percent of gross horsepower, and drawbar horsepower is around 75 percent to 85 percent of the PTO horsepower. A common mistake made by many agricultural equipment shoppers is that they compare one of these horsepower ratings to another, leading to false conclusions. The same type of horsepower needs to be used to make a decision for or against one tractor or another when holding comparisons.