This study aimed to compare the predictive power of grit and two cognitive ability tests of fluid and crystallized intelligence used for university admission on the success of college students in Turkey. Utilizing Cattell's Investment Theory and Ackerman's PPIK Theory of Adult Intelligence, we hypothesized that knowledge tests would be a better predictor of academic achievement in college than tests of fluid intelligence. We collected data from 441 students enrolled in engineering, physical sciences, social sciences, and administrative sciences majors in a technical university. Our results based on hierarchical regression and dominance analyses provided support for our hypothesis. For science, technology, engineering and math (STEM) students, the test of crystallized intelligence not only was a better predictor of college GPA compared to the test of fluid intelligence but also explained incremental variance over and above the fluid intelligence test. For social-administrative sciences, the predictive powers of tests were equivalent to each other. We also found that the perseverance of effort dimension of grit was the best predictor of GPA. Our findings support the notions of the adult intelligence theories suggesting that domain knowledge is a better predictor of typical performance in adults.