10mph = 4.47 m/s
0.3m Diameter (300mm) * PI = 0.94m circumference
0.94*4.47=4.75

Therefore the wheel needs to be turning 4.75 times per second to get 10 mph

Taking the motor speed at 5500 rpm
5500/60 = 91.7 revolutions per second

91.7/4.75 = 19.3

Therefore you need a gear ratio of about 20:1