Final answer:
The most fuel-efficient speed range for trucks is between 70-80 km/h as this range best balances speed with air resistance, thereby maximizing fuel economy.
Step-by-step explanation:
The most fuel-efficient range for any type of truck is considered to be between 70-80 km/h. This is because, at highway speeds, a significant amount of a vehicle's power is used to overcome air drag. Studies, such as those conducted by NASA researchers testing model planes in wind tunnels, have shown that over 50% of the power of a car can be used just to overcome air resistance, and the most fuel-efficient cruising speed turns out to be roughly within this range. Other references corroborate this, indicating that during the 1970s oil crisis in the United States, the maximum speed on highways was set to 90 km/h (about 55 mi/h) to promote fuel efficiency.