Final answer:
Cruising at 120 KM/H increases fuel consumption compared to 100 KM/H due to increased air resistance, which causes the car to use more power and thus more fuel. The most fuel-efficient cruising speed is around 70-80 km/h.
Step-by-step explanation:
Cruising at 120 KM/H can significantly increase fuel consumption compared to 100 KM/H. This is primarily because the force of air drag increases with the square of velocity. As a vehicle's speed increases, the power required to overcome this drag increases, and hence, its fuel consumption does as well. At highway speeds, over 50% of the power of a car is used to overcome air drag, which suggests that the most fuel-efficient cruising speed is about 70-80 km/h. Thus, exceeding this speed, particularly going from 100 KM/H to 120 KM/H, will result in greater fuel consumption.
The relationship between speed and fuel efficiency is complex and can be influenced by multiple factors including vehicle design, weight, engine efficiency, and aerodynamics. When weighing the driving option to use less fuel, one should consider that an efficient doubly-occupied car that gets 45 m.p.g. (5.2 L/100 km) can match the energy expenditure per person of a fully-occupied airplane getting 90 m.p.g. per seat.