The SUV, a 2009 Ford Explorer with a 4-liter V6 engine, was also tested while towing an enclosed trailer. The researchers tested the vehicles at a variety of speeds with the different configurations.

The results for the SUV pulling a 3,500 pound enclosed cargo trailer were significant, resulting in FE penalties ranging from 30%, for the city cycle, to 50% at 80 mph (129 km/h), at which point significant CO generation indicated protective enrichment due to high load.

The FE penalty associated with the rooftop cargo box mounted on the compact sedan was as high as 25-27% at higher speeds, where the aerodynamic drag is most pronounced. The compact sedan, a 2009 Toyota Corolla with a 1.8-liter four-cylinder engine, also suffered as its fuel economy dipped 22% from 42.5 mpg (5.53 l/100 km) at 60 mph (97 km/h) to 33 mpg (7.1 l/100 km), when hauling the rooftop cargo box.

Using a rooftop cargo box with the SUV decreased fuel economy from 24.9 mpg (9.45 l/100 km) at 60 mph to 22.9 mpg (10.3 l/100 km)—a drop of 9%.

At the other end of the spectrum, equipped with the cargo tray, the Corolla’s mileage at 60 mph was unaffected while the Explorer’s fuel economy decreased only slightly, from 24.9 to 24.7 mpg (9.5 l/100 km). A cargo tray is attached to the rear of a vehicle using a cargo hitch, about even with the bumper.

Other findings:

• Low tire pressure (50 and 75% of the manufacturer recommendation) resulted in negligible to 10% fuel economy penalties;

• Driving with all four windows down decreased fuel economy by 4-8.5% for the Corolla and 1-4% for the Explorer;

• The best fuel economies were achieved at a constant speed of 40 mph (64.4 km/h) for the Corolla with 57.5 mpg (4.09 l/100 km) and 50 mph (80.5 km/h) for the Explorer with 29.5 mpg (7.97 l/100km); and

• At 80 mph, fuel economy for the Corolla dropped to 30.9 mpg (7.6 l/100 km) while the Explorer dropped to 17.7 mpg (13.3 l/100 km).

Emissions from the vehicles were not significantly affected by the different configurations with the exception of the cargo trailer, which led to substantial increases in carbon monoxide due to protective enrichment, in which an engine under high load runs rich (higher fuel-to-air ratio) to protect the engine components and the exhaust system from the very high exhaust temperatures. This may happen, for example, when a vehicle is pulling a heavy boat up a hill.

While the findings were not unexpected, they serve as a reminder of how drivers can save money by taking simple measures.

There is fuel economy information and advice available for vehicle maintenance and carrying loads that is quite good, but very little published data to back it up. Certainly, suitcases strapped to your car’s roof and trying to keep up with a speeding Ferrari will adversely affect your gas mileage.

—John Thomas, co-author and member of ORNL’s Energy and Transportation Science Division.

Resources

• Thomas, J., Huff, S., and West, B. (2014) “Fuel Economy and Emissions Effects of Low Tire Pressure, Open Windows, Roof Top and Hitch-Mounted Cargo, and Trailer,” SAE Int. J. Passeng. Cars - Mech. Syst. 7 (2) doi: 10.4271/2014-01-1614