Jealous of that top-down roadster in the next lane? Take heart: Its driver will probably pay for it at the pump. Put the top down in a convertible and gas mileage takes a hit at cruising speeds — the result of an open cabin that’s anything but aerodynamic. The exact reduction is open for debate: One test found a slight mileage improvement at low speeds with the top down, followed by a mileage decrease at medium and high speeds.
I logged 100 miles of steady highway cruising in one of our test cars, a 2012 Mercedes-Benz SLK350, to investigate. According to the trip computer, losing the SLK’s folding hardtop cost 4.4 mpg — 27.3 versus 31.7 mpg. That’s significant, but probably not enough to deter any SLK driver from going topless on a nice day. (With the car, that is.)
Still, a 14% hit in gas mileage seems significant. I consulted James Smith, former president of the Society of Automotive Engineers, to find out why. Smith currently teaches mechanical and aerospace engineering at the University of West Virginia.
“The [auto] designers are making every attempt they can to make the air flow smoothly up the windshield, across the roof and down the back deck,” Smith told me. “And when you remove that roofline you have more area there; more room for the air to get recirculated back in.”
Not every top-down scenario is created equal.