If FDR remains the same after swapping pinion/spur, I would anticipate the motor position not changing.
I was wrong about this.
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FDR is related to total tooth count, and whether you're running 50/100, 100/50, or 75/75, you still have the same number of teeth and the motor position should remain the same
Additionally, there is no performance difference between one pinion/spur combo and another if the FDR remains the same.