Aerodynamic drag is only one factor in a long equation about the absolute driving resistance of a vehicle and overall efficiency depends on a lot of other factors. But even just considering aerodynamic drag, the way we compare cars doesn’t make sense at all.
The drag coefficient (Cd) seems to be a widely accepted metric for the efficiency of a car. When car companies introduce a new vehicle, they often mention this (low) number to communicate how efficient the new model is. Without more information, however, it tells us nothing about the absolute aerodynamic drag of the car.
The drag coefficient is always relative to the reference area (which should be the frontal area of the car). This means that without the size of the frontal area, we can’t tell what the actual aerodynamic drag is. One could advertise a car with a Cd value of 0.32 as more efficient than the previous model with say, 0.35, while the newer model could actually induce more drag due to having a larger frontal area.
I would suggest stopping advertising a car’s aerodynamics with the Cd value, but instead provide Cd × Frontal Area. Please?