Gear ratios are dependant on the motor specifications. Provided that the motor you intend to use has sufficient power for the application and you have a controller and battery pack that is also sufficient, then the 20mph is merely a function of matching the maximum motor speed to the tire size with perhaps 10-20% margin for the max. speed.
If motor speed max is 5000 rpm then 4500 rpm (10% less ) would be coupled against the rpm the tire has at 20 mph. At 20 mph * 5280 ft/mile = 105600 ft/hr.
105600 /60 = 1760 ft./min
A tire diameter of 1 ft = 3.1416 feet in circumference
1760 / 3.1416 = 560 rpm
4500 / 560 = a gear ratio of approx. 8 to 1