This publication looks at the relation of motor fuel characteristics to engine performance. In order to give satisfactory engine performance a motor fuel must have adequate partial volatility to enable the motor to be started and operated at the lowest temperatures found in the manifold. The dew point or temperature of complete vaporization should be low enough to prevent condensation on the cylinder walls, and high enough to prevent appreciable superheating of the mixture in highly heated manifolds. The vapor pressure should be limited in order to prevent gassing in the carburetor bowl or fuel ducts. The anti-knock quality of the fuel should be sufficient to prevent engine knocking and the accompanying loss in power and efficiency. Fuels meeting these requirements of volatility may be produced at no increase in cost by blending material having the desired volatility at low temperatures with other material having the desired dew points. Anti-knock qualities are generally improved by increasing the volatility but can be best obtained by selecting or treating the fuel specifically for this purpose.