2008 Buick Enclave: Roof Rack Redux
February 13, 2008
Our 2008 Buick Enclave was part of a recent three-car fuel economy investigation. I'm helping Phil Reed update the We Test the Tips fuel economy article that appears on the Edmunds.com website.
Anyone who's ever put their hand out the window of a moving car can surmise that putting stuff on the roof of a car hurts fuel economy. But how much? I don't want to give the farm away, but our test of driving with luggage on the roof produced a much more significant fuel economy penalty, in pure MPG terms, than we'd expected.
In order to back-up those results, I just finished repeating the comparison using our VBOX GPS data logger and a different method:a "coast-down" test. A coast-down is similar to a stopping-distance test, but instead of using the brakes you throw the transmission in neutral and coast. Aerodynamic drag provides the braking force. Oh, and you don't actually come to a stop.
That's OK because we're interested in the aerodynamic effects caused byroof-bound luggage while cruising on the highway. My coast-down test began at 75 mph and ended at something like 50 mph. I was most interested in the 75to 65 mphslice of the data. I averaged twoopposite-direction runs to cancel out wind and slope effects.Here are the results:
Bare roof: 75-65 coast distance= 1,235 feet; coast time = 12.0 seconds. Loaded roof (see photo): 75-65 coast distance = 1,024 feet; coast time = 10.0 seconds.
Coast performance was 21%better without luggage. The only variable here was aerodynamics,sothe dragdifference was the culprit. Drag is proportional to the mathematical product of frontal area and drag coefficient, and the presence ofluggage on a car's roof worsens both factors.
How did all of this impact MPG in our fuel economy test? I'll spill a few beans:we used exactly 21% less fuel with the luggage removed. Inmiles-per-gallon terms, the difference was significant: almost 6mpg.
Dan Edmunds, Director of Vehicle Testing @ 11771 miles