Thanks to the Second Law of Thermodynamics, no cycle or process is 100% efficient. Your car loses energy through:

• Heat escaping from the cylinders and the engine. Not all of the energy created in combustion is able to drive the car.
• Friction between moving parts converts some of the energy to heat which is dissipated to the environment. Keeping your engine well lubricated will help reduce this loss.
• Pumping fuel to the engine. Since your gas tank is beneath your engine, your car must expend energy to move it into the engine.
• Incomplete combustion. Your engine cannot completely combust all of the fuel during the cycle. Some of the fuel is not burned and is wasted.
• Braking. Energy is dissipated through heat thanks to friction.
• Wind resistance. Your car must push its way through the air in the atmosphere. This creates drag and makes your engine expend more energy to travel.
• Rolling resistance. There is friction between your tires and the road which dissipates energy as heat. Driving on well maintained, paved roads with properly inflated tires will help reduce your rolling resistance.
• Drivetrain losses. Energy is never transmitted 100% efficiently between parts of your car, so your engine must burn more fuel to compensate.

Authored by: Andrew Kristof