Forget the website, even the manual doesn't have it published.
Data from motor plate:
1800 RPM 162DC Volts @ 3 Amp
Mine was manufactured Nov 2006.
Zero RPM would be with switch off.
Switch on with variable resistor at lowest setting and belt on slow speed yields a spindle speed of 500 RPM - measuring pulley sizes top and bottom will yield motor RPM. Likewise, if motor is running at max of 1800 RPM and published hi speed spindle rpm is 3,900, measuring the pulleys should yield an advantage of 3900/1800 or 2.167. Variations can be many reasons such as old motor, bearing drag, different pulley size, etc.