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What is the effect on car fuel consumption of having a roof or not?

I guess it may depend on the car model, but generally, considering common commercial cars, which setting saves more fuel?

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Regarding aerodynamics, the back of the car is just as important as the front. While the front of the car is responsible for cutting though the air ahead of it, the rear is responsible for returning the air to the void that the front created.

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As seen in the above picture, when the air flowing over and around vehicle reaches the rear edge, it creates a light vacuum behind the vehicle, along with an abundance of turbulence and swirling air. This vacuum increases the aerodynamic drag on the vehicle. This is why airplanes taper to a point at the back, so as to decrease the drag on the rear of the plane. I don't have a source, but I believe the highest angle air can adhere to before separating and creating turbulence is 15°. While it would decrease the aerodynamic drag, it would be impracticable and perhaps a little dangerous for cars to taper to a point at the back.

This problem is exacerbated in a convertible, as the last edge for the air to conform is near the front/middle of the car, creating turbulence and causing drag along the entire back half of the car. The last point the air touches, the top of the windshield, creates an even larger vacuum from which to cause drag, as opposed to the sedan pictured, which tapers down to the height of the trunk before significant drag begins to occur.

Which factors have the greatest effect on mileage?

As the other answers/comments have stated, convertibles will usually be heavier than their coupe counterparts. The weight can be attributed to the extra strength required to raise the chassis rigidity that was lost when the rigid roof was removed, as well as extra weight from the motors and hydraulic systems needed to raise and lower the roof.

As user 'Imre' mentioned in a comment, the drag coefficient (Cd, stated as a ratio therefore has no units) of a convertible can vary significantly if the roof is in the up or down position.

Let's continue with Imre's example car: the 2011 Mercedes-Benz E550. The E550 coupe weights 3720 lbs (1690 kg) has a Cd of 0.24, and is rated at 15 / 23 mpg (city / highway). while The E550 cabrio weights 4160 lbs (1890 kg) has a Cd range from 0.28 to 0.38, and is rated at 15 / 19 mpg.

According to the United States Environmental Protection Agency (EPA), increasing the vehicle weight by 100 lbs (45 kg) will reduce fuel efficiency by 1-2%. In theory, the ~400 lb weight difference will only account for a loss of 1-2 mpg. Additionally, as Force = Mass x Acceleration, the excess weight should have a greater effect on city driving, as that is when the most acceleration occurs (as opposed to the constant velocity experience in highway driving), though the EPA numbers do no reflect this.

Paraphrasing Max Schenkel, General Motors Technical Fellow in Aerodynamics (source), "a change in drag coefficient of 0.01 is approximately equal to an improvement in fuel economy of .... approximately 0.2 mpg" .... "on the combined city/highway driving cycle". If you took the extreme example of the E550 Cabrio with the AirCap activated, the difference in Cd between the coupe and cabrio cars is .14, which could very much account for the difference in highway fuel economy between the two.

  • Nice. If you keep answering older questions you'll turn into a badge machine. – DucatiKiller Apr 6 '16 at 20:04
  • @DucatiKiller thanks, didn't even notice the date! I was motivated to answer after the discussion that just cropped up under Juann's answer. – MooseLucifer Apr 6 '16 at 20:10
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Modern cabriolets have similar consumption numbers with the roof up or down. But that being said, a cabriolet version of a vehicle model would normally weigh more because of all the extra reinforcement needed to stiffen the body in the absence of a stiff roof. This means that e.g. the VW Golf V 2.0 would weigh let's say 1400kg and the VW Golf V 2.0 Cabrio will weigh 1500kg. That means it has to carry around an extra 100kg, which WILL make for worse fuel consumption.

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    Where do you find consumption numbers separated for roof up or down? E.g. for 2011 Mercedes-Benz E-class cabrio, drag coefficient was cited 0.28 with top up, 0.33 top down and 0.38 with AirCap win deflection. I find it hard to believe fuel consumption stays constant despite different drag coefficient. – Imre Mar 18 '16 at 6:40
  • @Imre higher drag will definitely increase fuel consumption, you should convert your comment into an answer! – MooseLucifer Apr 5 '16 at 20:25
  • Drag doesn't really factor into it if you're not going very fast. – Captain Kenpachi Apr 6 '16 at 7:57
  • @MooseLucifer I was really just hoping to see some reference for the similar consumption claim. The core of my answer would be the same, weight difference. And according to instant consumption numbers on dashboard, drag does seem to make a noticeable difference only on road/highway - but I haven't yet bothered enough to measure this with any serious precision. In practical mind, a cabriolet won't make much sense anyway. – Imre Apr 6 '16 at 16:20
  • @JuannStrauss I think drag plays a larger role than you give it credit for, especially considering Imre's comment about the Mercedes Cd numbers. In the states, freeway/highway 60+mph (100 kph) driving is fairly common, if not a daily occurrence. I'm working on piecing an answer together that includes efficiency of drag vs weight. – MooseLucifer Apr 6 '16 at 16:46
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Typically if you're doing any kind of speed, roof up with be more aerodynamically economical although I personally didn't notice much difference on either of mine. That said, I didn't pay much attention to fuel consumption in either of those cars.

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