It took about dozens of links to find a data on diameter of sprocket for metal tracks on M113. Fortunately I found a post on scale models forum, where guy just went to garage and measured it on real machine 
Another issue I have is really weird one, somehow I can’t match speed of tread to speed of sprocket. This is like a basic physics.
- Calculate angular velocity of sprocket in radians, this comes from engine and other factors which influence tracks
- Turn it into degrees, multiply by DeltaT and call Add Local Rotation on sprocket mesh
- Then “sprocket angular velocity * sprocket radius” should be the linear velocity of the track. Radius can be scaled by few centimeters as tread is not placed on edge of sprocket. But anyway the result is not even close to what is expected. I was able to somewhat match the speed only when I assume that sprocket radius is about twice as small.
“W” is axle angular velocity
“AddLocalRotation( RadiansToDegree(W) )” is animation of sprocket
“W * SprocketRadius” is linear velocity of track.
But this doesn’t match visually,
" W * SprocketRadius * 1/2" somewhat matches it but not exactly, which doesn’t make sense.