Engine Efficiency: The specific output of an engine is a good indicator of engine efficiency. Specific output of an engine can be defined as the ratio between power and displacement. The higher the specific output in terms 01 PS/litre, the more efficient the engine is. Lower emissions are also a sign of a more efficient engine. For example, an engine designed to meet Euro IV pollution norms will be more efficient than one designed to meet Euro I norms. Fuel injected engines are more efficient than carbureted ones though this might not always translate into better fuel efficiency thanks to India’s preoccupation with exhaust emission norms preceeding everything else at the moment. Even an efficient engine might not translate into better fuel efficiency because it might be powering a much heavier car than an inefficienct engine.
Aerodynamics: Aerodynamics play an important part in fuel efficiency – an efficient engine might not result in a fuel efficient car if the coefficient of drag of the car is high. Generally, higher cars will offer more resistance than lower ones and even if their engines are the same, taller cars will consume more fuel than lower ones. As a rule, putting a luggage carrier on the roof of the vehicle will interfere with the aerodynamics and cause the car to return sub-optimal fuel consumption figures. And piling up luggage on the rack would cause a lot of drag depending on the frontal area created by the luggage – besides adding to the weight.
Gadgetry: Aids that drain power from the engine result in more consumption of fuel. That is true of air conditioning and hydraulic power assist for the steering