[autoavia]

As you reduce the voltage to a motor you will also reduce the torque which the motor produces so it will be much less progressive. You will eventually find a point where the voltage is too low to allow the motor to turn at all.

One solution is a adjustable controller like a Slotit or Truspeed but this is quite expensive. As Grah1 has already said another much less expensive option is a lower revving motor.

It might help if you posted more details about the car that is causing problems and what track, power supply and controller you are using and explain exactly why you need to slow the car down..