Study: How To Improve Mileage New study By Applying Newton's Law To Better Fuel Economy

For drivers looking to improve gas mileage on their summer vacations there’s just no getting around Sir Isaac Newton. The famous 18th century physicist built some impressive calculations about objects in motion, and those equations apply as much to today’s vehicles as they did to the horse and buggy.

According to a new study by the Oak Ridge National Laboratory, drivers can cut fuel economy in half just by the way they drive and how they pack their vehicle.

“Certainly, suitcases strapped to your car’s roof and trying to keep up with a speeding Ferrari will adversely affect your mileage,” said John Thomas, co-author of the study and member of the ORNL’s Energy and Transportation Science Division.

Most of the results confirm long-held suspicions about towing trailers or boats, carrying rooftop items, speeding and driving on underinflated tires. Researchers used a variety of configurations on a sports utility vehicle and sedan.

“There is fuel economy information and advice available for vehicle maintenance and carrying loads that is quite good,” Thomas said, “but very little published data to back it up.”

Thomas’ work puts the heft of science behind anecdotal evidence. Terms like drag co-efficient, force curves and the “NASCAR phenomenon” casually roll off Thomas’ tongue the way parents discuss their children’s homework.

Thomas: “Two different force curves affect the car at different speeds.”

Translation: Vehicle weight and speed are the persistent devils of fuel economy. The more you weigh down your vehicle, the less mileage it gets. And the faster you driver with a heavily laden vehicle, mileage decreases rapidly.

As an example, SUV drivers pulling a trailer at 80 MPH can expect to lower fuel economy by as much as 50 percent. Thomas:

“The force of aerodrag is proportional to the velocity squared.”

Translation: The faster you drive, the harder your engine works. Air resistance goes up and fuel economy goes down. Passenger vehicles are not intended for the racetrack. They are designed to provide the most comfort and economy for typical city and freeway driving.

Thomas cited what he calls the “NASCAR Effect,” where stock cars traveling at speeds more than 200 get extremely poor mileage.

“A sedan traveling at NASCAR speeds will only get a few miles to the gallon,” Thomas said.

That natural law of physics has a nice side benefit for bored children on a lonely stretch of freeway. Extend a flattened hand outside the window and you can feel the effect of “lift”—the same force that lets planes leave the runway.

Passengers should never stick an arm or hand outside a moving vehicle, Thomas emphasized. Moreover, the ORNL study proved rolling down a vehicle’s windows on the freeway also reduces gas mileage. Results of the ORNL were released in early April and sponsored by the Department of Energy.

The study utilized a 6-cylinder sport utility vehicle and four-cylinder compact sedan. Researchers tested vehicles with different configurations such as underinflated tires, open windows, and rooftop and hitch-mounted cargo. They also conducted tests at a variety of speeds.

Here are some of the study’s results and Thomas’ suggestions for improving mileage on your next family road trip:

• Driving with all four windows down decreases fuel economy by as much as 8.5 percent (depending on vehicle type and engine size).• Towing a 3,500-pound enclosed cargo trailer resulted in fuel economy penalties ranging from 30 percent in city driving to 50 percent at 80 mph for the test SUV.
• The best fuel economies were achieved at a constant speed of 40 mph for the test sedan with 57.5 mpg and 50 mph for the test SUV with 29.5 mpg,At 80 mph, fuel economy for the test sedan dropped to 30.9 mpg while the test SUV dropped to 17.7 mpg.
• Low tire pressure (50 and 75 percent of the manufacturer recommendation) can reduce fuel economy by up to 10 percent.
• Hybrid vehicles speeding down the freeway operate on their gas engines and not battery energy. As a result, fuel economy rules apply the same as for traditional gasoline-powered vehicles.
• Boxes and suitcases strapped to top of a vehicle increase wind resistance. Consider using an aerodynamic cargo carrier.
• Hitch-mounted carriers tend to offer less wind resistance than other types of racks since air flows over the back of the vehicle. “Anything that has a lot of aerodynamic drag is going to reduce fuel economy,” said Thomas, who emphasized driving carefully, “There’s no way to improve your fuel economy by going faster and faster.”