The Institute's dynamic ratings of good, acceptable, marginal, or poor are derived from two seat design parameters (peak acceleration of the dummy's torso and time from impact initiation to head restraint contact with the dummy's head) plus neck tension and shear forces recorded on BioRID during the test. (iihs.org)
Shorter ratios would mean higher peak acceleration in each gear, though probably not a faster GT4. (roadandtrack.com)