Two identical blocks of mass $m$ rest on a frictionless flatbed boxcar. In the boxcar's frame, block 1 moves toward block 2 (initially at rest) at $4$ m/s; after their inelastic collision, block 1 has speed $1$ m/s. The boxcar rolls along a straight track at a constant $4$ m/s relative to the ground, in the same direction as block 1's initial motion. What percentage of the system's original kinetic energy, as measured in the ground frame, is dissipated by the collision?