I was planning on rigging things up like this:
The motor is rated at 500kv. If I power it with an 18.5v Li-po pack, then the motor should spin at ~9,000 RPM's.
I'm not going to use a gear box. Just a small chain or belt drive system. Not sure about the gearing and stuff like that...
A 1:10 gear ratio - 900 RPM would give a maximum speed of ~10mph.
A 1:5 gear ratio - 1,800 RPM would give a maxium speed of ~20mph.
So I'd probably gear it at about a 1:4 ratio or so. That way I could keep the motors RPM's down while cruzing at 10-15mph.
Thoughts?