6V, 12V, 24V power wheels cars are common in the market, there are also 36V & 48V faster electric cars for kids age 10 and up. Speed of kids riding toys:
6V kids car: about 2-3mph
12V ride on car: about 4mph
24V ride on toys: about 6mph
36V power wheels for big kids: about 15mph+
48V electric cars for 10-year-olds to drive: about 18mph+
The size of the battery and the size of the motor are two determinants of the speed of the car. It is worth noting that the size of the battery and the size of the motor need to match. This means that 12V electric motors need to be used with a 12V battery. If the motor and battery do not match, the motor burning due to the excess power, or the electric vehicle cannot move due to insufficient power.