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Wax on, wax off
June 29, 2011 9:05 AM   Subscribe

Does a clean, waxed car REALLY increase fuel efficiency?

It's been over 10 years since I took fluid dynamics, and I keep hearing that a clean, waxed car will increase your fuel efficiency (there were recently some "Good Cents" radio spots on my local NPR station extolling the virtues of waxing your car). Something just doesn't feel right to me about that assertion. I don't deny that a smoother surface generates less friction, I just don't think that the average driver (not racing driver) would really see an efficiency benefit from keeping their car freshly waxed.

(Assume an average sedan with some dirt, but no bird nests or piles, during highway and city driving)
posted by Kronur to Science & Nature (11 answers total)
 
Car Talk took on this question a couple of years ago.
posted by proj at 9:19 AM on June 29, 2011 [3 favorites]


Yes, but you're unlikely to see it to any significant degree. To explain:

There are two types of drag. "Bluff body" drag is the effect of essentially changing the momentum of the air you're passing through; think of parachutes. Aerodynamic drag is a side effect of generating lift, and is usually less than the bluff body drag for something not designed to generate lift, e.g. your car. An average sedan will see more effects from "bluff body" drag rather than aerodynamic drag. Dirt and other roughness has much more of an effect on aerodynamic drag than simple body drag. My out-of-the-air guess is that you'll see maybe 1% difference on your fuel economy if you're very thorough about washing and waxing.

Where you will see significant effects based on roughness are certain categories of airfoils. The most notable use of these were during World War II - the Mustang was pretty notorious for suffering significant performance degradation if they weren't meticulously cleaned and waxed.
posted by backseatpilot at 9:35 AM on June 29, 2011


Car Talk took on this question a couple of years ago.

So did Mythbusters. Although technically what they were testing was "Dirty car vs Clean car", so not specifically "waxed". Their findings were that the random distribution of dirt particles caused drag, reducing efficiency. If you wish to make the jump to "waxing reduces the random particle distribution even more, ergo even BETTER efficiency," you go right ahead. So, by combining the Mythbusters and Car Talk results, you should probably keep it clean, but the waxing is just for show.
posted by JJtheJetPlane at 9:36 AM on June 29, 2011


The Mythbusters were able to show that a dirty car got 2 MPG worse mileage than a clean one, although technically the myth they were testing was the opposite (seriously guys, wtf?). However, to get that result they used a seriously dirty car, with a layer of crap sprayed on so thick that you couldn't see anything through the windows, and their testing was at a constant 65 MPH where wind resistance dominates. Obviously they chose these conditions to make the difference as stark as possible, but in a more realistic scenario where you burn significant amounts of gas at non-highway speeds where air resistance is less of a factor and where the dirt coating doesn't resemble a spray-textured ceiling, the difference would be less. Also, waxing was not addressed at all.
posted by Rhomboid at 9:38 AM on June 29, 2011


I don't think so.

The air on the surface of a moving car doesn't actually move; it has a laminar flow. This means a thin layer of air adheres to the car, and air flows in parallel layers over this so-called boundary layer. (If you want to sound fancy, you could refer to it as the Blasius boundary layer.) Of course your side mirrors and antenna create a turbulent flow...

But you know how you can drive on the highway and still have a fine layer of pollen on the car? You can easily wipe it off with your finger, but the wind doesn't blow it off because of that boundary layer of air. Waxing shouldn't make any difference.

On preview: if the car has a surface equivalent to a spray-textured ceiling, I'd guess you're not going to get any laminar flow at all...
posted by Specklet at 9:46 AM on June 29, 2011


You get better gas milage by going 65mph instead of 70 then by making sure your car is clean all the time.

Also the cost of keeping the car clean all the time might outweigh the 25 cents maybe it costs in fuel.
posted by majortom1981 at 10:20 AM on June 29, 2011 [3 favorites]


To sum up; yes, you would see a theoretical increase in fuel efficiency in a clean and waxed car. But that increase would be lost in the noise under real-world conditions so on a practical level the answer is no.
posted by Justinian at 2:27 PM on June 29, 2011 [1 favorite]


I'm not sure if water acts the same way as air, but I have found that there is some benefit to be gained from getting rid of the gloss surface on the bottom of race boats using ~800 grade wet-and-dry sandpaper and this is fairly common practice. The theory is that it reduces surface tension. Although the science on this is probably dubious, there is a measurable (although tiny) improvement in speed.
posted by dg at 3:25 PM on June 29, 2011


Most likely the guy with the waxed and shiny car gets better gas mileage because he keeps what's under the hood up to snuff, too.
posted by BlueHorse at 10:31 PM on June 29, 2011


I should have known to check the Car Talk guys. My googling efforts seemed to mainly bring up noise. Thanks everyone!
posted by Kronur at 5:37 AM on June 30, 2011


My car always *feels* faster when it is looking pretty, but that's probably the extent of it.

The aerodynamics of a car are complicated. There are some flow patterns where a rough surface might be better, because the air flowing over the roughness introduces a swirling effect which creates a ball bearing effect and allows the larger "blob" of air to flow more freely. (Think golf ball. The divots in the ball catch little balls of air and make it go farther.)

I believe there have been tests that show that a pickup truck with an open bed gets better mileage with the tailgate shut because that creates one of those pockets of air. (Obviously, a covered bed is better.)

There is another thing that is more micro, which is those little spoilers you see on some cars that are on the roof a couple inches ahead of the rear. (called vortex generators) They introduce a little turbulence in the laminar flow that causes the air to change the shape in which it travels over the car. (Or wing. Last diagram.) By spoiling the effects of any sharp turns the laminar flow has to make, you reduce the "footprint" the object has in the static air.

Here is a pdf that explains it.

In other words, some cars might be better, some might be worse. The optimum solution would be for it to be dirty in some areas, and shiny in others. To the extent that the dirt effectively created beneficial vortices.
posted by gjc at 7:42 AM on June 30, 2011


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