Help! Vehicle Collision Impact Speed

You can calculate the length of both:
sqrt( x*x + y*y + z*z ) = length

thats basically the speed in your example, but...

if you want to calculate the force, you should get the dot-product between the normalized velocities and the normalized position-difference (during collision):
norm( pos1 - pos2 ) = diff
dot( - diff, norm( v1 ) ) = strength1
dot( + diff, norm( v2 ) ) = strength2

and multiply it with the speed/length value (and maybe with a object-mass).

Maus


btw: http://www.gamedev.net/topic/625147-car-collision-speeds/