Cd ratings are often supplied by the manufacturers. Lower Cd ratings indicate less air resistance. Along with the Cd rating, check out the frontal area of the vehicle. This is the size of the vehicle when viewed from the front. A car with a large frontal area and a low Cd rating will still have more air resistance than a car with a small frontal area and a slightly higher Cd rating.
Small cars weigh less and have smaller engines.
Lighter cars use less fuel getting the vehicle up to driving speed and smaller engines require less fuel to operate. Again, think small. Buying a new car is not an option for some drivers but they too can increase their fuel economy. This happens because most drivers modify their driving habits during this driving period. Drive the car as if there was a raw egg placed between the accelerator pedal and your foot.