Since gas prices have topped $4 a gallon, tips on saving gas have been the flavor-of-the-day in the media. But does driving slower really save gas? If it takes me longer to get there, isn’t my car burning more gas?
The short answer is that slowing down does increase the fuel economy of your vehicle. The reason lies in aerodynamics. What you need to know is this: As speed doubles, the force (aerodynamic drag) on your vehicle increases fourfold. For example, a house built to withstand 200 mph wind has to be four times stronger than a house built to withstand 100 mph wind. The formula for aerodynamic drag on your vehicle takes into account how aerodynamic it is, what the area of its cross section is, what the density of the air is, and what your vehicle’s speed is. Since you can’t influence the density of the air, and you can’t change your vehicle’s aerodynamics, that leaves only speed. And slower speed equals less drag.
According to theÂ Green Car Co., you can decrease your fuel use by 20 percent by slowing down from 70 mph to 60 mph. Keep in mind that driving 10 mph faster over a 30-mile commute will only save you about four minutes. There is, of course, a limit. Driving 5 mph will not earn you a Hero of the Planet Award.
Engines are designed to be the most efficient in a specific range of RPMs (rotations per minute), typically around 3,000 rpm. Accelerating quickly requires more RPMs and more gasoline. This is why most nonhybrid cars have a higher EPA mpg rating for highway driving than city driving. Frequent stops and starts at red lights or stop signs require a lot more energy than cruising at a constant speed. Some cities, especially in Europe, have reduced vehicle emissions from stops and starts by converting intersections to roundabouts, which allow vehicle traffic to flow smoothly around in a counterclockwise circle.