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.
Most convertibles have windscreens, and some employ deflectors above the windshield to reduce wind noise and intrusion. All those measures can pay dividends in fuel efficiency.
“The noise is an indicator that you’ve disturbed the air, and if you’ve disturbed the air, the energy has to come from somewhere,” Smith said. “If you can cut the noise down, the odds are you’ve also cut the drag down, because it’s the turbulence that you’re hearing. And there are attempts to decrease the noise level, which is decreasing the turbulence level.”
Are any convertibles actually designed for better aerodynamics with the top down? Smith doubts it and he suspects the EPA probably doesn’t account for top-down driving in its test cycles. (EPA officials didn’t answer my emails.)
“If you’re a producer of a vehicle, you’re going to maximize fuel economy for the condition the car is in the most,” he said. “And the odds are the car will have its top up the most.”
I used cruise control and air conditioning most of the way for both legs — 50 miles apiece on Michigan’s Interstate 96 westbound — and encountered light headwinds (5 to 10 mph, according to Weather.com) and temperatures in the high 80s. I logged data only for steady-state cruising, cutting out portions to exit the interstate, deploy the roof and reaccelerate to highway speeds. Other route notes:
- Top down: Windows up but no sunscreen, or Mercedes’ optional AirGuide wind deflectors, deployed. Average speed: 69 mph. Trip-computer mileage: 27.3 mpg
- Top up: Windows up. Average speed: 73 mph. Trip-computer mileage: 31.7 mpg