The Calculus Diaries - Jennifer Ouellette [24]
Let the naysayers knock my plucky little hybrid if they must, but thanks to that real-time graphic display, I am now hyperaware of how much energy I consume when driving, and how much even tiny changes in driver behavior, type of terrain, or weather conditions can affect my overall mileage. By virtue of constant feedback on your fuel-efficiency performance, the Prius trains you to be a more energy-conscious driver. For instance, accelerate gradually, and you’ll use less energy than if you put pedal to the metal in a vain attempt to go from 0 to 60 in a few seconds.
Also, traveling at a steady speed, even in heavy traffic, is better than jerkily starting and stopping, because every time you restart after a full stop, you have to overcome the car’s inertia all over again. I try to leave a bit of extra distance between my car and the vehicle just ahead, so I can coast a little rather than brake suddenly. Under the best conditions, the difference can be as significant as getting 75 miles per gallon versus 25 mpg. I reflect on that whenever I feel frustration at Los Angeles’ notoriously congested freeways. I might be inching along at a snail’s pace, but I reap the benefit by averaging many more miles per gallon, even if it takes longer to reach my destination. Collectively, these practices have become known as hypermiling.
Even traveling at a steady speed, in general, the faster you go, the more energy it takes to maintain that speed because of increased air resistance (drag). The engine has to work constantly to overcome the resulting drag and thus consumes more fuel. It’s tough to correctly calculate the drag coefficient for anything but the simplest of shapes, but in general, at high speeds, the drag force increases as the square of the velocity. In plain English, this means that if you’re traveling at 100 mph, you’ll experience four times the drag force you’d experience if you were traveling at 50 mph.16
Small increments in improved fuel efficiency can add up significantly over time. So driving just at (or slightly under) the speed limit can result in considerable energy savings in the long term. Back in 1974, the federal government instituted a 55 mph speed limit on highways, not because it was safer17 but because it conserved fuel at a time when oil was scarce. Similarly, driving uphill uses more energy than coasting downhill—any avid bicyclist could tell you that—as does driving into a strong headwind. Certain driving conditions are beyond one’s control. Don’t even get me started on what a ten-hour drive from Salt Lake City to Los Angeles in gusting crosswinds through a mountain pass did to my average miles per gallon.
Why doesn’t everyone ditch their current gas-guzzling cars for a Prius or similar hybrid? The answer might surprise you. It turns out that many of us assume that saving gas (and therefore money) corresponds linearly with miles per gallon. But according to a June 20, 2008, article in Science by Richard Larrick and Jack Soll at Duke University’s Fuqua School of Business, the gas used per mile is actually inversely proportional to miles per gallon. They call this the mpg illusion.
Let’s say you own two cars: one with a 34 mpg rating, like Sean’s old Toyota Corolla, and another with an 18 mpg rating, like my father’s beat-up Chevy pickup. Should you replace the 34 mpg Corolla with a pricey 50 mpg hybrid, or the 18 mpg pickup for a cheaper 28 mpg nonhybrid vehicle, in order to achieve optimal savings? You want to optimize those gas savings to recoup your initial capital investment as quickly as possible. Run the numbers, and it becomes apparent that replacing