In order to better isolate the causative influence of mass (as distinct from other factors such as size) on driver risk in two-car crashes, Newtonian Mechanics is used to establish that when two cars crash head-on into each other, the ratio of their changes in speed (delta-v) is inversely proportional to the ratio of their masses. Using National Accident Sampling System (NASS) data, the probability of a driver fatality is determined as a function of delta-v. Combining these two effects, the risk of death in the lighter car divided by the risk of death in the heavier car (the "fatality risk ratio") is calculated as a function of the mass of the heavier car divided by the mass of the lighter car (the "mass ratio"). Parallel analyses are performed for severe injuries (AIS ³ 3), and for belted drivers as well as drivers irrespective of belt use. The calculated relationships between fatality risk ratio and mass ratio are of the same form as those determined in an earlier study using actual fatality data. This agreement suggests that mass, the only vehicle factor in the calculations, is the dominant causative factor in the large influence of mass ratio on fatality risk ratio. Fatality and injury risk ratio increases with increasing mass ratio more steeply for belted drivers than for unbelted drivers, implying that when all drivers use belts the relative disadvantage in the lighter car becomes even greater. The effect of car mass on injuries, while large, is still substantially less than on fatalities, showing that relative risk results obtained for fatalities should not be presumed to apply, even approximately, to other injury severities.