Originally Posted by sohc_mshue

Hp is the same regardless of gearing only torque changes. The dyno will take the input speed(i.e. roller speed for a roller dyno), and take the torque and figure out hp vs. roller speed. If it knows the engine speed often suplied from an inductive pickup or gear ratio calculation then it can use the measured hp, and engine speed and output a torque curve vs. engine speed. If it doesn't have a proper tach signal or gear ratio calculation then it will simply give the wrong torque numbers, but hp will be the same.