Brighter Planet

Did you know your car’s fuel economy varies depending on the speed at which you drive? Different vehicles achieve maximum fuel economy at different speeds, but on average mileage increases until 25-30 mph, stays about constant until 55-60 mph, and then declines rapidly. Why? At that speed air resistance starts to overcome even the most streamlined design, because the power required to overcome the force of air resistance is proportional to the cube of velocity. Your car needs about 15 horsepower to maintain speed at 60 mph, but that jumps to 120 horsepower at 120 mph.

Ever wonder why your speedometer has a line at 55 mph? In 1974 congress passed the National Maximum Speed Law which—you guessed it—set a national speed limit of 55 mph. The law was intended to conserve gasoline in response to the 1973 oil crisis. The 55 mph limit was modified in the 1980s, and finally repealed in 1995. It’s actually a coincidence that modern cars often reach their peak fuel economy at about 55 mph—back in 1973 the average peak was at 40 mph, but no-one was willing to set the speed limit that low!

The upshot of all this is that slowing down can really save money and carbon. Each 5 mph you drive over 60 mph saves about 7 minutes on a 100-mile trip, but costs the equivalent of paying an extra 25 cents per gallon for gas. You’ll also emit an extra 1.5 lbs of CO₂ every 20 miles. For the average American, the difference between driving at 75 mph and 65 mph — or 70 mph and 60 mph — adds up to almost $300 and 1,400 lbs of CO₂ each year.