Ask Pablo

Follow these simple tips and you can drive your sky-high gasoline bill back down to earth.


Pablo Plastic
July 14, 2008 3:20PM (UTC)

Dear Pablo,

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.

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According to the 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.

Rolling resistance is another force that your car must overcome. Imagine pushing your car along a level street. As you begin to tire you will quickly understand how powerful your car is. If you were to let some air out of your tires you would see the rolling resistance increase. This is why one of the most common fuel economy boosting tips is to make sure that your tires are inflated to the proper level.

Reports say that sales of fuel-economy-boosting gadgets and fuel additives are up drastically this year. Keep in mind, if it sounds too good to be true, it is! Special magnets on your fuel line will not allow for more efficient combustion. Adding an acetone-based additive may boost fuel economy by a small amount but is not worth the added expense and potential for damaging your engine. And, no, your car will not run on water, despite what the spam e-mails might tell you.

If you're in the market for a new car, especially one that gets good gas mileage, don't forget to consider aerodynamics. A teardrop-shaped car will get better mileage than your neighbor's shoe-box-shaped transporter. On an open-ended scale that measures aerodynamic drag, where zero is the most efficient, the sleek Honda Insight scores 5.10, while the Hummer H2 scores a pathetic 26.3. As for your current car, you can improve aerodynamics yourself. Tips from the car site Edmunds include:

  • Reducing the use of roof racks
  • Rolling up your windows and turning on the air conditioner at higher speeds, typically above 35 mph
  • Replacing a broken or missing front air dam
  • Lowering your vehicle
  • Running narrower tires
  • Choosing smoother wheels (ideally, flush discs like those on vehicles trying to set land speed records)
  • If those sound too technical, here are some quick and easy tips to improve your fuel economy, whether it is to save money, the environment or both:

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  • Observe the speed limit
  • Use your cruise control
  • Avoid routes with a lot of stop signs and red lights
  • Avoid unnecessary hills
  • Remove excess weight from your trunk
  • Check your tire air pressure frequently
  • Carpool or take public transit
  • Combine errands (remember, the most efficient drive is the one avoided)
  • Ride your bike or walk to the store at least once a week

  • Pablo Plastic

    Got a question about the environment? Ask Pablo at AskPablo@Salon.com.

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