This work was performed to investigate how the likelihood of a two-car crash depends on the driver age and car mass for each of the two involved cars, and also to examine the special case of cars of similar mass crashing into each other. Data on 108 044 cars involved in police reported two-car crashes occurring in New York State in 1971 and 1972 were fitted to a function of the driver age and car mass for each of the two involved cars. Car registrations are used to estimate exposure. The special case of crashes between cars of similar mass is considered because of prior results on driver injuries in such crashes. It is found that "small-small" crashes (defined as a 900 kg car crashing into another 900 kg car) are about 0.3 times as likely as "big-big" crashes (an 1800 kg car crashing into another 1800 kg car), assuming equal numbers of cars driven by drivers of the same age. Combining the present results with earlier findings of increased injury risk in small-small crashes gives that such crashes injure about 70% as many drivers as big-big crashes when normalized for numbers of cars and driver age. That is, it is concluded that small-small crashes produce 30% fewer injuries than do big-big crashes.