Students compare their achievement in a subject with their classmates' achievements (social comparison), their own prior achievements (temporal comparison), and their achievements in other subjects (dimensional comparison), which can each be better (upward comparison), equal (lateral comparison), or worse (downward comparison). Prior research has investigated the impact of different comparison motivations on the prevalence of social and temporal comparisons, but no study has examined the same for dimensional comparisons yet. The present study closes this gap: A total of 605 German high school students were presented with four situations, in which a fictitious student receives the same objective feedback for an exam in a certain subject, but is motivated either to evaluate, to enhance, to improve, or to differentiate himself. For each comparison motivation, the participants judged how likely the fictitious student was to draw dimensional, social, and temporal upward, lateral, and downward comparisons. As a central result, dimensional comparisons in all directions had the highest prevalences under the self-differentiation motivation. In contrast, the prevalences of dimensional comparisons were relatively low under the other three motivations. This finding complements the recently developed dimensional comparison theory. For the first time, we could empirically show that dimensional comparisons primarily serve self-differentiation motivations.