Linear Velocity

The rigidbody has the velocity of a given game object. If you want the projection of one vector onto another then what you are looking for is the dot product

so for c#)...

float speedForward = Vector3.Dot(rigidbody.velocity,transform.forward);

which assumes this is a behaviour on a gameobject with a rigidbody...

these pages may be helpful:

http://www.mvps.org/directx/articles/math/dot/index.htm http://unity3d.com/support/documentation/ScriptReference/Rigidbody-velocity.html