control
what you need is a simple form of a control system.
First figure out a way to convert your car's speed into voltage. Another poster mentioned a tach, and that may be a good way of doing it.
Now - let's say you want your car to move at 10 cm/sec, and lets say this corresponds to 5 volts when converted to voltage using a tach circuit.
This means you need a 5 volt reference.
Here's the basic version of the circuit you'll need. Now you have two signals - your 5 volt reference, and output from the tach. You take an op-amp, and connect your 5 volt reference to the positive input, and the output from the tach to the negative input. You connect the ouptput of the op-amp to the drive of your motor.
What happens is this:
a) your car is running at the desired speed (in our case 10cm/s), which means that the tach produces 5volts, and both inputs of the op-amp are equal, and you get 0 at the output (meaning you don't need to apply power to your motor, because you are already going at the desired speed)
b) your car is going too fast (let's say 15cm/s), which means that the tach will produce, let's say 7.5 Volts. Now you have 5 volts reference connected to the positive input, and 7.5 volt reference connected to the negative input - which means that the output of an opamp will be negative - thus slowing down your motors, because you are going to fast.
c) your car is going too slow (let's say 5cm/s), in which case your tach produces 2.5V. Your reference is 5Volts, and you input 2.5 Volts to the negative input of an opamp - which means the output will be positive, thus increasing power to your motors, and speeding up your car.
This is the basic version of a control system (in fact you are using proportional version of the controller). If you want to go deeper into the whole thing, you can do PI controller (proportional plus integral), or PD (proportional plus derivative). And if you have plenty of time you can even do PID version (proportional, integral, and derivative). Integral lets you maintain constant speed more closely, and derivative allows you to adjust the speed faster. Look up control systems on-line to find examples. For your particular case I'd recommend starting with proportional, and doing integral later on. Also - don't use amplifier in open-loop configuration - apply negative feedback.
Hope this helps.