Speed Sensors

[lukevanoort]

If you use encoders of some sort and have the weight/space I'd suggest mounting them on a non driven wheel that touches the floor to eliminate error from the robot doing spinning its wheels.

[mluckham]

Accelerometer doesn't tell you current speed, it just measures acceleration. And if you are turning at the time, you will get the forward component of acceleration plus the lateral component (2-axis accelerometer).

Using encoders, you can figure distance travelled, speed, and acceleration.
Put an IF statement in your code that executes its inner code at a known rate (eg: 10 times per second).

if (time_elapsed_100msec)
{
   Speed = CurrentEncoderCount - prev_EncoderCount;
   Acceleration = Speed - prev_Speed;

   prev_EncoderCount = CurrentEncoderCount;
   prev_Speed = Speed;
}