There are several equations you need to be familiar with when dealing with hydrostatic drives. Two of the more basic ones are:
GPM=RPM*displacement/231 (displacement is cubic inches)
and
torque=PSI*displacement/6.28 (torque is in-lb and displacement is cubic inches)
Using these equations you can determine the maximum flow rate from your pump at full displacement then the resulting motor speed. By knowing your relief valve setting you can then determine the motor torque.
Moresmoke noted the HP=GPM*PSI/1714 relationship which is also important. This comes into play because you want a system that can move enough oil at a high enough pressure to fully load your engine. Your available 16hp would not be usable if you're blowing over relief when only pulling 10hp from the engine.
If you currently have a fixed displacement motor the main thing to understand is that you essentially have a single speed transmission. Regardless of the displacement of the hydrostatic pump the resulting maximum motor torque will be the same so the chosen motor size becomes a compromise between speed and torque. There is really nothing you can do with this setup to duplicate the speed and torque range that a multi-speed transmission provides. The only way this can be overcome is with a variable displacement motor or inserting a multi-range gearbox between the fixed motor and drive axle.
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