Here's a repost from the info page:
4T: 631rpm / 48v = 13.16 kv ( measurement by me, adjusted down 1 volt )
5T: 485rpm / 48v = 10.1 kv ( fluxshifter's measurement )
6T: 336rpm / 48v = 7kv ( assumption based on 3kV steps per winding )
Rough idea of 48v speeds in a 26" wheel:
4T: 39mph ( observed )
5T: 30mph ( calculated but not observed )
6T: 21mph ( assumed )
The Kv is roughly how many RPM's you'll get per applied volt, so at 66V the common 5T version would be 726-RPMs.
IF...you mount it in a 26-inch wheel and lift it unloaded to spin in the air, 726-RPMs would be 56-MPH. Any speed over 30-MPH will experience a large effect from wind resistance, so I suspect you'd likely achieve roughly 45-MPH without any aero-fairings (deducting 20%). I may have made an error somewhere in the calcs, but that should give you a rough idea.
"Wheel diameter to speed/RPM chart"