Getting better gas mileage by reducing air drag

A lot of us at Rocky Mountain Institute lead active lifestyles. Whether it's riding single track in summer or heading for the ski lifts in winter, we spend a lot of time hauling around outdoor gear.

We also spend a lot of time thinking about the efficiency of our vehicles.

Which got me thinking: Just how much extra fuel am I burning by keeping my roof rack on year-round, adding extra drag to my car?

And, more importantly, can I save money and reduce my carbon footprint?

The basic science
It turns out that air drag is the single largest factor affecting fuel consumption while driving on level ground at normal highway speeds. 

The faster you go, the worse it gets, because drag increases exponentially with speed.  Driving twice as fast quadruples your drag, and the amount of power required to overcome that resistance increases by a factor of eight (cube law: 2³=8). 

Don't believe me? Think about what it feels like to stick your hand out the window at 80 mph vs. 40 mph. At the higher speed, you're paying to overcome the added resistance by burning more gas.

Improving performance
To improve your vehicle's aerodynamics, the primary solutions are to decrease speed, frontal area, and turbulent airflow. 

Some people go the distance to customize their vehicles for improved drag performance. Check out this rather extreme example of "boat-tailing."

Fortunately for the rest of us, there are far simpler (and more socially acceptable) ways to boost your mileage by paying attention to the air flowing around your car.

Here are some simple things to keep in mind next time you get behind the steering wheel:

Your speed
First, slow down!  Fifty-five mph may be too slow for your taste, but staying in the 60-mph range could save you quite a bit of money.

According to the Department of Energy's fuel economy Web site, driving at 80 mph is equivalent to adding roughly $0.80/gallon to the price of your fuel versus driving at 60.

Roof racks
Racks are great for getting weekend gear to the trailhead, but costly on the daily commute. It's best to use them when you need them and take them off when you don't. I've got my swap-time to less than 5 minutes.

Most sources claim a roughly 5 percent drop in fuel economy from roof racks; from personal experience, I’ve measured about a 10 percent drop. If you take the racks off of the car for half of your driving, you'll save an average of 15-30 gallons of gas per year. If you swap them 12 times per year, that can work out to over $100/hour for your efforts. Who knew aerodynamics could be so lucrative?

If that sounds like too much of a hassle, a wind-deflecting fairing for the front rack costs about $50 and will do a good bit to reduce drag and noise. The wider the better: try to find one that smoothly bridges the gap between the windshield and the front bar. 

A model that improves efficiency by even 3 percent will pay off the investment in around a year of average driving (assuming 15,000 miles/year, 27.5 mpg, $3.50/gallon).

Other options
Any time you have the option of carrying gear behind rather than on top of your vehicle, do it. 

Trunk-mounted bicycle racks are typically much cheaper, and offer substantial savings compared to carrying your bike on the roof.

On a recent 440-mile road trip to Moab, I found that having one bike on top of my car cut my mileage by 25 percent. That meant I burned almost 4 gallons of fuel just to move the bike!

On vehicles equipped with hitches, using a rear-mounted cargo box in place of a rooftop box will save lots of gas -- and be easier to access. 

If you have a rooftop box, for the sake of your wallet and the planet, take it off in between uses.

Most importantly, though, enjoy the time outdoors!

Aaron Westgate is Special Aide to Rocky Mountain Institute co-founder and Chief Scientist Amory Lovins.