To increase wheel speed, you can:
-reduce the gear ratio
-get bigger wheels
-increase voltage to the motor
THey all have disadvantages. THe most notable ones are you lose torque with the first two so you can't accelerate or climb as well, and with the third method you use more power, heat up the motor more, and might exceed motor specs.
Of course, you can also get a new motor altogether. Faster motors are a lot faster, but may not have enough torque. You can also get an equally fast or stronger motor and combine it with any of the methods above.