Using change in magnitude of accelerometer to calculate speed?
Your question didn't specify what velocity you're trying to measure, so I'll answer for multiple versions of this question.

First off, integrating acceleration to get velocity leads to drift. This is why [nearly] everything that uses an accelerometer for position/velocity information (and cares about accuracy) also has a GPS to remove those errors. Integration will always lead to errors, though they can be managed.

A better approach is to ask yourself what you're really trying to measure.

Angular Velocity:

Use something better for measuring angular velocity, like an optical encoder (as @kolosy mentioned) or a magnetic sensor (like bicycle speedometers). These each have their pros and cons, but they directly measure distance vs. time and thus directly give you velocity.

Linear Velocity:

Depending on the velocities and precisions involved, use a GPS (if you're going to be moving at a constant rate most of the time and will be going relatively fast), or use an angular-velocity measurement method on a wheel (which never exceeds the limits of static friction with the ground) and multiple that by its circumference.

In short:

Don't integrate unless you have to.