A case-emphasized propulsion method improves vehicle fuel efficiency. The ratio of the most-used speed to the most-desired speed of an IC engine is employed to control the engine to always run at its optimal working state with an efficient single-stage gear transmission. The propulsion method used with different brands of IC engines demonstrates a reduction in fuel consumption between 5 and 39%. An n-ratio automatic single-stage gear transmission implements the propulsion method. The transmission design executes the proposed propulsion method as well a continuous transmission, but it can also increase the propulsion efficiency about 8 to 18% when applied to replace traditional automatic transmissions or continuously variable transmissions in vehicle drivetrains.