How do I compare the HP ratings of an old tractor with new tractors?

The ratings given for older tractors were developed by the Nebraska Tractor Tests under a stringent set of guidelines (these are found in the foreword of "Nebraska Tractor Tests from 1926"). Since the rules imposed on manufacturers for real farm tractors are not imposed on smaller homeowner machines built currently, the HP ratings quoted on modern brands are not comparable. Current ratings are similar to the rating of a lawn mower measuring only raw crankshaft horsepower. It has been suggested that a factor of 1.5 applied to a Nebraska rating will provide equal HP but even this will not take into account the application of the weight required to use the HP. The HP rated on the older machines measured usable power at the drawbar and at the PTO.