Shifting vehicle feet composition in the United States and United Kingdom, and concerns relating to vehicle compatibility and occupant safety, prompted investigation of the respective mixes of passenger cars and light trucks in two-vehicle collision. Differences between striking and struck vehicle mass for frontal and front-to-driver’s side car-to-car and light truck-to-car crashes on US and UK roads are examined, as is US data on relative importance of mass and vehicle size parameters that address geometric compatibility in influencing fatality odds. Findings suggest that significant disparities between vehicle fleets in the two countries warrant further study before direct comparisons between safety experiences are made.