You have clearly misinterpreted the Mythbusters' data. They did not suggest that a dirty car got better gas mileage than a clean one. To the contrary, the reverse was true.

I have an explanation for your change in mileage. From reading many of your posts, I understand you have a 20 mile or so course you mapped out that is apparently ideal for maximizing fuel economy in the FEH with your driving style. On that course you report you are capable of achieving 70+ MPG. You have, therefore, the ability to manipulate and inflate your tank average MPG by the number of times you drive that course or one similar to it. If average tank MPG starts dropping, you can go drive your course and pump it back up again. So just seeing your average MPG tank data is a bit misleading without knowing the driving you did to achieve it.

To be clear, there is no way, no how a dirty vehicle gets better MPG than a a freshly cleaned and waxed (or sealed) one, all other things remaining equal. One possible exception I guess would be if your dirt happened to accumulate in a dimpled pattern.

 

The Mythbusters show had a few test with mud and dirt placed on the car and called it dirty. I don't mean to imply that's how I drive my FEH and dirty to me is the normal need for a wash. Cleaned and waxed is where I see the drop in MPG. The theory of the Mythbusters show I thought, answered the reason I get a drop in mileage when I wax my FEH's. Prior to the show all I knew was after waxing I saw a drop in MPG. The car Mythbusters used already had a decent front end wind resistance but still had a good amount of drag from the rear of the car at high speeds. Our FEH has a much larger amount of friction and turbulence in the front as well as a large amount of drag from the rear. In my mind, the mud and dirt added to the car just increased the wind resistance of that already decent front end wind resistance car, and decreased the overall MPG at high speeds. It appears to me the dimples had and overall better effect at reducing drag from the rear of the car and was the cause of the increased MPG at high speeds.

My interpretation of the Mythbusters show was that rear drag was the bigger issue reducing MPG than the front end friction and turbulence. This helped me understand why a wax job was decreasing my mileage. I thought the wax reduces the friction and turbulence on the front end and therefore increased the drag on the rear end. With just washing my FEH, I see no improvement in MPG, but I see no decrease before it needs a wash. Letting my FEH get normally dirty over a coat of wax slowly took my mileage back to normal for me. You can see that from my mileage log. You can also bet I was driving my hardest to maintain my tank during those two ~55mpg tanks. According to you, the wax job should have given me better mileage.

I think I've made it clear I plan my routes to get the best mileage. I do have a commute I take from Jupiter to the WPB Courthouse which is about 16 miles and I make stops at my bank, Costco, Sams, Walmart, and I often pick-up my wife at her office and go to lunch. When I'm pressed for time I'm not afraid to get on I-95 because I know I can make-up any lost mileage on other portions of roads with 30 - 35mph limits along my normal commute downtown and back. Of course, no matter where I drive I find bad conditions and great conditions but always use hypermiling techniques. I do get many cases to drive longer distances in our business to pick-up or drop-off work and yes, I can get great mileage using coastal routes.

Most of my driving is in EV range so being able to tell how my FEH is affected by air flow is very easy. I just pulse to a target speed and glide in EV. As I've said before, the glide distance and the drop in speed is a dead giveaway of how airflow affects your MPG. You keep on washing and waxing your FEH if you think that gives you better gas mileage, no wax for me tho.

GaryG

 

I'm finding myself not very convinced. If a "dirty" surface actually results in less air friction -- enough to make a testable result in MPG -- then we would be seeing textured surfaces on aircraft wings and fuselages. Instead they go to great expense to file down all the rivets (as opposed to old planes like the Ford tri-motor) and create a laminar airflow.

Irregular surfaces generate turbulence. Turbulence takes energy to create.

I'm not disputing the data -- the theory to explain it just doesn't fit.

 

Prior to seeing Mythbuster, all I knew was waxing reduced my ability to get the best mileage on a round trip. I couldn't explain why, but I knew the results was clear. You all have a FEH, but no one including Gator can tell us what mileage they had before a wax job and after. I think that's because none of you could truly tell the difference. At a 4% decrease from 57mpg, I see a 2mpg drop where most you would only see about a 1.2mpg drop and not think anything about it. I'm most likely the first to make this claim and maybe I should have kept it to myself from the response I've gotten.

I've clearly demonstrated my ability to drive the FEH in public for the best MPG during the '07 Hybridfest MPG Challenge. It is my ability to tell what helps or hurts my mileage to get up to a 58mpg 790 mile tank on flat roads. I just read a ~2 year old post on CleanMPG.com about washing and waxing to get better gas mileage. Only two people answered the question and they both said waxing does not make a difference. One of the posters was the World record MPG setter and owner of the site, Wayne Gerdes. He also indicated he had not washed his car in a long time. And BTW, the second place FEH winner behind me at the '07 MPG Challenge also got an award for the dirtiest vehicle there, it had mud all over it!

GaryG

 

You said it! It's just a claim that hasn't been verified. Just because GaryG, who had best MPG during the '07 Hybridfest MPG Challenge says so doesn't make it true. I only mention that because you always mention it.

I have to agree with the poster above that mentioned the surfaces of planes. I don't see textured helmets or suits on cyclists either.

 

So Bill or anyone else, prove my claim wrong! No one here or anywhere else has claimed they get better mileage in their FEH after washing or waxing.

GaryG

 

Nope! You made the claim! You must prove it and so far, you haven't, it just a unproven claim!

 

Don't really want to wade into this debate. But I thought it worth noting that I've heard about controversy in the swimming world over the bodysuits that folks have been breaking lots of records with. I believe they're modeled on sharkskin and have some texture. I also believe that they're getting more restricted on coverage in the future. Different fluid, similar mechanics.

My understanding is that it's based on boundary layer conditions. There's always going to be a stagnant layer next to the surface (car, airfoil, swimmer) as it passes through the fluid (air, water). By increasing that boundary layer with texture, you're really moving the friction interface to air-with-air or water-with-water. My guess as to why airplanes aren't textured is that they're moving at much-higher velocities to the point where you can't improve the boundary layer without adding significant drag (or cost effectively, which is probably more likely).

 

What about NASCAR, Indy, etc. at much higher speeds but still use a smooth body?

 

Well I guess there is something to this after all. From the page:

http://www.school-for-champions.com/...ging_fluid.htm

Turbulence factor

An interesting aspect of fluid friction is that setting up tiny areas of turbulence on the surface will reduce the friction even more. A good example of that are the dimples on the surface of a golf ball. If an ball had a completely smooth surface, it would not fly as far as an identical ball with a dimpled surface. The reason is that each dimple creates a small area of turbulence on the surface. This means the air is then flowing over air in that area and not along the surface of the ball, thus reducing the resistive friction.

Dimples on golf ball reduce surface friction

But note that if the dimples were too large or deep, then the turbulence would cause the ball to slow down. Dimple dimensions and locations are very important in the design of golf *****.