You can't directly use an accelerometer as a speedometer. You have to integrate the acceleration over time to get the change of velocity; if you accelerate from a known velocity (eg at rest - no velocity) at a known acceleration for a known period of time, you can calculate the velocity you will be travelling at.
If you don't have a known starting velocity, you can only determine how the velocity has changed since you started to measure the acceleration.
It's not particularly accurate and prone to drift over time. Even precision accelerometer systems (such as those used for inertial navigation systems) will drift over time and need to be re-referenced either by starting again from a know state (eg still and facing a known direction) or supplemented by a different measurement system (eg GPS).