Hi phoobarnvaz,
about 98% of the Q-Matic owners use the 56 tooth sprocket.
About 1% use the optional EZM 60 tooth sprocket.
The remaining 1% use sprockets from 62 to 72 teeth.
If you have hills and or extra weight we suggest from 60 to 66, as the larger sprockets above 66] allow the motor to rev too high. The few riders that use the LARGE sprockets either pull a trailer, ride at average speeds of 25 MPH, or know not to run the motor at RPM levels above 8000.
I will list the speed numbers @7800 RPMs [red line] to help you decide on the correct sprocket for your use.
44 tooth 50 MPH [not possible, due to motor load]
56 tooth 39 MPH [not likely, due to motor load]
60 tooth 36 MPH
62 tooth 35 MPH
64 tooth 34 MPH
66 tooth 33 MPH
The stock Q-Matic uses the 56 tooth, and normally runs out of power at higher speeds [designed to hold top end speed lower to meet most state laws].
As the rear sprocket is increased the estimated speeds will be more likely to be true, as the LOAD is less and the motor is more likely to rev higher.
Have fun,