If you do not use the shifter kit, with the gearing it is a trade off between speed and torque. Smaller sprocket will give you more speed but you lose low end torque. Bigger sprocket will give torque but you sacrifice speed.
Most kits come with a 44t rear sprocket. After break in you can see speeds from 25-30mph,. I get a top speed of 32-33mph with mine but others say they can go faster.
With my 44t sprocket I have gone up a 9,000ft mountain with maintaining 18mph with some very step grades.
YouTube - Motorized Bicycle up Mount Lemmon in Tucson,AZ
I have added a 36t to my bike and get reach 37mph but I have to pedal to reach that and lost lots of low end torque. A 40 or 41T is a perfect medium between the 44 and 36 but I have yet to try one.