The power to overcome air resistance increases roughly with the cube of the speed, and thus the energy required per unit distance is roughly proportional to the square of speed. Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h), results in about one-third the power to overcome wind resistance, or about one half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy by 1.9 times, and drastically decreases fuel economy. In practice, rather than doubling or halving the fuel economy, the difference is actually closer to 40-50%, because engine efficiency varies greatly with the torque/speed operating point. Rolling resistance, which is broadly proportional to speed, is also a factor particularly at lower speeds.
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