Curves are presented from which it is possible to obtain the adiabatic compressibility of a petroleum oil in the range of 40°–200°F and up to 5000 pounds per square inch when only the specific gravity at 60°F and the kinematic viscosity at 100°F are known. A chart is given for obtaining the velocity of a pressure wave in an oil in which both the viscous drag and expansion of the containing tube are neglected. A table showing compressibilities and wave velocities for a number of Diesel fuel oils at 120°F and 1750 pounds per square inch pressure is given. The effects of viscosity and tube wall expansion on wave velocity are discussed and a chart is presented to aid in determining the wave velocity in steel tubes. The paper is concluded with a discussion of the effect of wave velocity on the point of injection of the fuel in the Diesel cycle.