I always wondered when some said that the diesel engine is more efficient than gasoline one ... this is what I've learned doing some research for my future boat and truck engine choices.

Engine Thermal Efficiency

Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work.

Traditionally gasoline engines are in the 25-30% range, meaning 70-75% energy is wasted as heat. Newer (last 3-5 years) gasoline engines with GDI (gasoline direct injection) increased the efficiency of the engines up to 35%.

Traditional diesel engines are in the 40% range (with RPMs up to 2000). Modern (last decade or so) turbo-diesel engines using electronically controlled common-rail fuel injection increases the efficiency up to 50% with the help of turbo-charging system. This also increases the engines' torque at low engine speeds (up to 2000 RPM).

Apparently, diesel engines are ahead of gasoline engines in efficiency advancements ... I will take 50% over 35% on a sound principle alone. Keep in mind, California is doing everything possible to make gasoline engines even more efficient.