I designed it for 40km/h max, but this is way too fast.
Given the KV rating of the motor and your max voltage you can calculate the max rotational speed of the motor = KV*V. From there it is just a matter of multiplying by all gear ratios along the drive train to calculate the rotational speed of the rear wheel. Knowing the circumference of the 26" wheel gives you the max speed.
Given the KV rating of the motor and your max voltage you can calculate the max rotational speed of the motor = KV*V. From there it is just a matter of multiplying by all gear ratios along the drive train to calculate the rotational speed of the rear wheel. Knowing the circumference of the 26" wheel gives you the max speed.