Mileage; Mythbuster?

[Whoops]

All right, I would like some help.

My wife told me that she was watching a program where they claimed that washing your car and waxing it was something people should do, to get better gas mileage.

She said the program claimed that their is a 7% improvement in mileage, if you wash your car and keep it waxed.

I would not doubt that their might be a improvement, but their is no way that I can believe it would be 7%. I think the article was sponsored by Turtle Wax?

Does anyone have any ideas of what this figure might really be?

[Shawn D.]

Aside from removing large chunks of mud, washing and waxing will get you essentially nothing other than a psychological "improvement" in MPG. Cars are affected almost entirely by pressure drag, not friction drag. Washing and waxing does nothing for the former, but can be significant for the latter. Any improvement would probably be less than half a percent.

[jesse.rizzo]

In my aviation classes we learned that a layer of frost the thickness of sandpaper on your wings will increase drag 40%, a fairly significant amount. Of course that would be a heck of a lot of dirt to get a layer that thick. Not to mention that drag increases exponentially with velocity, and the planes we fly cruise at around 120mph.

So I would venture to say that it would help, but not even enough to pay for the washing and waxing. My car is pretty dirty right now, maybe I'll test it out.

[Shawn D.]

Originally Posted by jesse.rizzo

In my aviation classes we learned that a layer of frost the thickness of sandpaper on your wings will increase drag 40%, a fairly significant amount. Of course that would be a heck of a lot of dirt to get a layer that thick. Not to mention that drag increases exponentially with velocity, and the planes we fly cruise at around 120mph.

So I would venture to say that it would help, but not even enough to pay for the washing and waxing. My car is pretty dirty right now, maybe I'll test it out.

Yes, BUT as I said -- friction drag on cars is negligible. Friction drag on aircraft is NOT negligible. It's not the same situation.

[ebacherville]

it really is amazing how much resistance picks up over say 60 mph.. riding a motorcycle really educates you on the increases of wind resistance in faster speeds.. cant even imagine these land speed motorcycle guys..

[cfg83]

ebacherville -

Originally Posted by ebacherville

it really is amazing how much resistance picks up over say 60 mph.. riding a motorcycle really educates you on the increases of wind resistance in faster speeds.. cant even imagine these land speed motorcycle guys..

Hmmmmm. *If* I was interested in the best MPG at 70 MPH (65 MPH Max + 5 MPH over), then maybe a wash and wax would just barely begin to help?

CarloSW2

[cookie96civic]

It must be right, I rinsed the sawdust off my car, easily a 9-12 mpg improvement. Oh yeah, looks like the water must have freed the e-brake too, sweet.

[Bror Jace]

"drag increases exponentially with velocity"

That's the key. Spend lots of time at or above 100mph? Then washing and waxing becomes crucial. I'm always amazed when I look at older aircraft with exposed rivets, etc ... what a waste of efficiency!

[Volones]

I don't think you'll see a noticable difference after washing/waxing.

I just got my best tank ever (44+), and my car is VERY dirty.

[justpassntime]

Volones,

Can you send me some Colorado dirt for my car?

[Shawn D.]

Originally Posted by Bror Jace

"drag increases exponentially with velocity"

That's the key. Spend lots of time at or above 100mph? Then washing and waxing becomes crucial. I'm always amazed when I look at older aircraft with exposed rivets, etc ... what a waste of efficiency!

Apparently, a number of you are disregarding what I said about pressure and friction drag on cars, maybe because I have no longstanding reputation here and haven't built up credibility. I really dislike pulling out this "hammer" because folks reflexively call it arrogant, but here it is: I'm an aerospace engineer and know what I'm talking about. There, I said it.

As I said before, pressure drag predominates on cars, not friction drag. Waxing does not become "crucial" for cars at any speed. In fact, having a rough surface enhances attachment on objects with low length/diameters, such as cars. Thus, waxing your car could make drag worse. In any case, the change is negligible.

As to the exposed rivets, it depends on the aircraft. It cost a great deal extra to manufacture it with a smooth surface, especially 40-60+ years ago. The P-38 was very smooth overall because the design led to thin boundary layers (i.e. no long fuselage), but the back end of a B-52 has many exposed fasteners/rivets, which isn't as important because the boundary layer was thick back there. Also, many combat aircraft were expected to have short operational lives due to attrition or obsolescence, so the extra $$$ was not always deemed worth it, especially when many had to be pumped out quickly.

[Bror Jace]

Shawn D, can you rig up a couple quick charts showing the influences of pressure and friction drag at different speeds ... maybe one for "typical" aircraft and another for a "typical" car? Just enough detail to show where they come into play.

Nothing too fancy ... X axis in gradations of 50mph ... Y would be % maybe. You could even draw this with MS Paint, I'd think.

[Shawn D.]

Originally Posted by Bror Jace

Shawn D, can you rig up a couple quick charts showing the influences of pressure and friction drag at different speeds ... maybe one for "typical" aircraft and another for a "typical" car? Just enough detail to show where they come into play.

Nothing too fancy ... X axis in gradations of 50mph ... Y would be % maybe. You could even draw this with MS Paint, I'd think.

Thanks for fashioning your response that way -- it's refreshing not to have to defend my background!

That would be a great concept if it were not for the fact that pressure and friction drag vary in lockstep with the square of velocity, so the percentage will stay the same throughout the speed ranges we're concerned with.* Note that measures of Cd make no distinction in what type of drag it is because short of doing CFD (computational fluid dynamics), you really can't tell what component is contributing what percent of the total, and again, the percentage for each remains the same. Aircraft also have an "induced drag" component which varies with speed and weight, but that isn't applicable to cars/trucks.

Sorry if this seems to be a non-answer, but it is the answer!

*Things change as one goes through the transonic region (starting at ~0.8 Mach) and into the supersonic region, but that's called "compressible flow," which is another realm altogether! Our land-borne vehicles doing under 600 mph are well into "incompressible flow."