Does a convertible get better gas mileage when the top is up?

More aerodynamically smooth when the top is up?

Thank you.

It would depend on the material of the car’s cover. Mercedes’ hard top convertible would probably be more aerodynamic with the top up, but a rough canvas, or other flexible material, wouldn’t be as aerodynamic, so on a car with a soft top, it would probably be a wash.

However, if you have to ask, a convertible probably isn’t for you. Convertibles are for people with a carefree attitude, and they usually disregard issues of fuel economy. If you care so much about fuel economy, you aren’t carefree enough to drive a convertible.

Robert where you you come up with these questions? If you have a convertible (I do) you don’t give a ---- about mpg on a nice “top down” day. The mpg difference isn’t significant. Top up or down depends on temperature and rain - not mpg.

If you want to get technical, it depends on the model of convertible. My '04 T’bird is very quiet with the top down indicating little aerodynamic disturbance and likely no change in mpg.

It is speed dependent. If you are cruising under 40 MPH, top up or down won’t affect mpg that much. Over 50 mph will start to make a difference.

At 100 mph you’ll get better mileage with the top up.

Are you thinking of trading the Expedition?

I just have never understood the charm of a convert. Windy, cold, can’t hear, easy to break into.

Must keep thExpedition for blizzards.
Yes, at 100 mph I bet it would also be quieter.
(Can’t we quote anyone anymore?)

Correct, I do not deserve a convertible.

“it would depend upon the material of the car’s cover”.
The overall shape, yes. But the very slight roughness of the texture of some material has affect in fluid dynamics then you may think and may actually improve the cars efficiency in some cases. New Zealand may have had a distinct advantage using a rough coating of the hull in America’s cup racing.

…then why do hard car top carriers have less wind drag than soft car top carriers?

Simply, they(hard carriers) maintain their aerodynamic shape better. They differ dramatically in that respect. No one is saying that smooth is not better than rough…but the difference I feel is secondary to shape and practically imeasurable by the layman at speeds we normally drive. The maximum difference in flow over boats amounted to 7 boat lengths per mile; significant in racing but I don’t feel that it’s that significant when translated to cars. For years, cars had textured roofs (simulated leather) as options.
And though gas mileage was not the issue then, you would have been hard pressed to measure a difference between those and hard tops.

If the surface were dimpled as a golf ball is dimpled, would the surface be more aerodynamically slippery?

Only if it were spinning while moving through the air which gives it lift. If your car were spherical and tumbled as you drove, it might benefit from dimples. :=) But, those dimples could also cause the dreaded slice with your car pulling to the right as you drove…that is if you’re a right handed diver.

On a golf ball, the dimples cause the air flow to become turbulent instead of laminar and turbulent air follows the contour of the ball farther before it separates.
Laminar flow is lower skin friction but separates more easily which results in more drag. If the body is shaped so that flow doesn’t separate, then laminar flow is lower drag.
As for fabric, a lot of airplanes have been covered in fabric, including the control surfaces of a lot of WWII fighter planes.
Here’s some interesting reading on the subject.

" The maximum difference in flow over boats amounted to 7 boat lengths per mile;"

And all you have to do is sail into a hole, sail the wrong side of a wind shift, or blow a tack and that lead is gone.

Dag is right about the golf ball. It’s the reverse spin and the turbulance created by the dimples that create the lift via a higher pressure under the ball than above. Unless, of course, you slice or hook. Then the same principle makes the ball curve and hit a tree.

As to whether a car is more slippery with the top up or down, it depends entirely on the car and the speed. It’s been written by the car mag guys that the Mercedes SL is so quiet with the top down that you can carry on a normal conversation at highway speeds. It’s also been writtten by the same guys that the 370Z is so noisy at highway speeds that you can’t hear yourself scream in pain. Convertables range all the way from the Nissan cross-vehicle to the Mazda Miata (RX-5). Every one has its own aerodynamics.

Air flow is a complex event. That is why auto manufacturers all have wind tunnels. It is much easier to just measure it than to compute it. :slight_smile:

Only if it were spinning while moving through the air which gives it lift. If your car were spherical and tumbled as you drove, it might benefit from dimples.
I had to come on here and see if this topic had ever been discussed after seeing the last half of a repeat episode of Mythbusters that tested this very premise.

The original myth they were testing was whether or not a dirty car got worse mileage than a clean car. I didn’t see that part of it, but I gathered that there was no appreciable difference. However, they took it a step further and covered the car with a fairly thick layer of clay – 800 pounds worth – and had sculptors sculpt it to exactly match the original contours of the car.

The way they tested was to put a small fuel cell in the trunk of the car (which was weighed before beginning), and accelerating the car to 65 mph using the normal fuel tank. Once they crossed the start line, they switched the fuel source to the fuel cell. Once they crossed the finish line, exactly one mile down the road, they switched off the fuel cell, and weighed the fuel cell again. After establishing the control mpg, they cut golf ball-like dimples into the clay and repeated the experiment. Their result was that they got slightly over 26 mpg with the smooth surface, and 29 mpg with the dimpled surface!

This would suggest (if accurate) that the spinning of the ball is irrelevant to the aerodynamic effect of the dimples.


Top Up

The dimples removed weight from the extra they added on it, so yeah, shedding several pounds from the car’s weight would have an effect on the MPG. Remove ALL the clay and see how many MPG they got


  1. Mythbusters = good TV, but not necessarily good science. Good science would have been done in a wind tunnel, where all variables including environmental conditions could be controlled.
  2. what worked on the mythbusters car, assuming it was a truely accurate result, would not necessarily work on another car…or even a different wind direction or wind speed. Aerodynamics is complex.
  3. whatever the result of the Mythbusters test, they cannot be applied to an entirely different shape like a golfball.
  4. the subject of golfball aerodynamics, dimples, and even the shape and size of the ideal dimple, has been extensively analyzed by the golfball manufacturers under controlled conditions, followed by real world testing. A simple Google will turn up tons of data.