Mark – there’s several factors that affect differences between engine, drawbar and PTO HP ratings: Transmission type: Simple gear drive transmissions are the most efficient – they lose only a few % of the engine flywheel HP passing thru them. Powershifts don’t do quite as well. Hydrostatics are the worst – may lose as much as 15% - 20% of the engine’s flywheel power. PTO design: Depending on the mechanical design PTO output may be limited to some fraction of the available engine HP. Example: I’m uncertain of the exact numbers but recall working several years ago with an older Steiger equipped with a hydraulic PTO (PTO motor-driven from the tractor hydraulics). The engine was rated at 325 HP, but the PTO was limited to 120(?) HP. Doesn’t necessarily mean it’s an inefficient design – only that the PTO mechanism is incapable of safely transmitting full engine HP. Tires: Same as with cars, older (bias ply?) tires lose more to HP to slippage and internal flexing than do modern tractor radials. This can amount to a difference in several % in drawbar HP all other things being equal. Weight: As with any vehicle, the heavier it is the more power is required to move it forward. So if a tractor is overballasted during the drawbar test HP will unnecessarily be be lost simply moving the tractor along (though the effect of this one will be relatively minor overall). So simply comparing published HP ratings without consideration to stuff like the above CAN be misleading!
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