Focal adhesions are large protein complexes organized at the basal surface of cells, which physically connect the extracellular matrix to the cytoskeleton and have long been speculated to mediate cell migration. However, whether clustering of these molecular components into focal adhesions is actually required for these proteins to regulate cell motility is unclear. Here we use quantitative microscopy to characterize descriptors of focal adhesion and cell motility for mouse embryonic fibroblasts and human fibrosarcoma cells, across a wide range of matrix compliance and following genetic manipulations of focal adhesion proteins (vinculin, talin, zyxin, FAK, and paxilin). This analysis reveals a tight, biphasic gaussian relationship between mean size of focal adhesions (not their number, surface density, or shape) and cell speed. The predictive power of this relationship is comprehensively validated by disrupting nonfocal adhesion proteins (α-actinin, F-actin, and myosin II) and subcellular organelles (mitochondria, nuclear DNA, etc.) not known to affect either focal adhesions or cell migration. This study suggests that the mean size of focal adhesions robustly and precisely predicts cell speed independently of focal adhesion surface density and molecular composition.