I just saw this article on CNN the other day: Planes slow down to save fuel. Apparently airlines are finally starting to feel the pinch of higher gas prices, and are learning that going at a slightly slower speed can save significant money.
This isn’t a new observation: young folks today probably aren’t aware of the fact that the former national 55 mph speed limit was originally implemented in 1974 not for safety, but to conserve fuel in the wake of the 1973 oil crisis.
The reality is that air resistance becomes a major limiting factor on fuel efficiency at high speeds. Roughly speaking, the drag force increases as the square of the velocity at high speeds, which means that driving 100 mph results in 4 times the drag force that driving at 50 mph causes.
I drive a hybrid Civic, and the dynamic mpg gauge on the vehicle gives me significant insight into how air resistance kills fuel efficiency. I typically find that, driving steady at 55 mph, I can get 40-50 mpg. Once I push the car up past 70 mph, and into 80 mph, I’m lucky to get 35 mpg. At low speeds, the car can ‘coast’ for appreciable distances without using fuel, but at high speeds, the engine must constantly work against the drag force.
These numbers, over long periods of time, add up: driving a little slower on a day-to-day basis can save you some significant money in the long run.