Example you have two 48 volt motors:

1. Generates 6 HP @ 3200 RPM with 9.8 ft-lbs of torque.

2. Generates 6 Hp @ 10,000 RPM with 3.2 ft-pounds of torque.

So which motor do you use. Well you use the one that generates the most power at the maximum engine RPM required for top speed. So if your target speed is 30 mph. you would need to know the motor RPM, differential ratios, and tire diameters to work that out.

You are correct in thinking higher voltage = higher RPM's as a general rule. HP is just like electrical power watts. Watts and Horse Power are products of Voltage and Current for watts, and RPM and Torque for HP.

I can get say 6000 watts with:

**6000 Watts = 1 Volts x 6000 Amps**turning a motor at 1 RPM with 31,512 ft-pounds of torque. You could pull a train a few inches per minute

or

**6000 watts = 6000 volts x 1 Amp**. Turning a motor at 6000 RPM with 5.3 ft-lbs of torque. making your bike go say 30 mph with the right transmission system. That is assuming that is enough power to overcome weight, rolling resistance, and COD.

So you could pull a train with one motor, o rmake your bike go 30 mph with a speed motor, but forget about pulling a train with it.