We have analyzed the relationship between P wave velocities, measured at pressures of 40, 100, and 200 MPa, and modal mineralogy in oceanic gabbro samples from Ocean Drilling Program Holes 735B, 894G, and 923A. At all pressures, increasing velocities correspond to increasing pyroxene contents and decreasing alteration (phyllosilicate and amphibole content) but indicate little or no variation of olivine content, in part because the olivine compositions are in the range Fo65 to Fo73. A Voigt-Reuss-Hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low, even relative to Fo73, possibly because the olivine grains contain ubiquitous networks of cracks. Even if the cracks are not present in situ, on average, gabbros with velocities typical of seismic layer 3 (6.7--7.0 km s-1) contain 5--15% alteration products, including 5--15% amphibole and 0--5% phyllosilicates. These results suggest that the lower crust is, on average, slightly to moderately altered.