  Physics dictates that cars with small mass will travel more miles per gallon (mpg) compared to massive trucks. Does this imply that small cars are more efficient machines? In this work a mileage efficiency metric is defined as a ratio of actual car mileage (mpg) to the mileage of an ideal car. This metric allows comparison of efficiencies of cars with different masses and fuel types. It is as useful to quantify efficiencies of cars as the concept of drag coefficient is to quantify the efficacy of their aerodynamic shapes. Maximum mileage and lowest CO2 emission of conventional gasoline cars, at different driving schedules, is reported based on the concept of an ideal car. This can help put government imposed standards in a rigorous context. 
Mileage efficiency and relative emission of automotive vehicles