Tank Tracks

I want to do the Wall-E in blender, this popular robot can change the “shape” of his tracks.

For close the track he change the distance between parts of track, so how can i do it? In array modifier is an option [fit curve] which add next segments to fill the curve but I dont wanna add next segments.

How can i change the Relative Offset to fill the curve automatically?

You either need to add segments or stretch the tread-- the length of the curve changes.

If you want to stretch the tread, you could do this with tread along a curve (not with fit to curve, just arrayed and scaled to eye), and give the curve stretch in properties/object data. Then anything using it for a curve modifier will scale automatically to completely fit the curve.