DIY aerodynamics

Julian Edgar changed the underside of a 1999 Prius from this:

to this, with a fitted sheet of ABS plastic:

He claims a 10% increase in MPG at highway speeds.

looks possible, but it you could get a 10% increase by adding a $5 piece of steel or plastic, I would think Toyota would have added this a long time ago.

VW Passat’s have a similar cover, except it extends further back. Problem is that you have to remove it to do an oil change, and after you do this a number of times, the screws holding it in place don’t hold any more. I’ve had two of them break off at speed, not fun. From what I gather, most Passats have the cover missing by the time they are 8 years old or so.

Also, you have to be careful that you don’t block the cooling of some component. The VW has some openings in the cover to direct air flow to certain places.

Yeah, I’m skeptical, too. I was expected a much bigger revision. Toyota worked years on the aero, surprised if it could be improved so easily.

Accurate MPG measurements are extremely difficult, as it’s impossible to hold the conditions constant. some of the variables are highway, temperature, speed, drivers mood, etc.

Also, the drivers expectations can color the results. As evidence, witness the large number of people who bought into that scam about using battery power to electrolyze water and injecting the “brown gas” (H2 and O2 mixture) into the air intake. Hundreds of people swore they had an increase in MPG using this scam.

“Accurate MPG measurements are extremely difficult…”

The author used a relatively large sample base, which can prove how repeatable his measurement procedure is.
Repeatability is more relevant than absolute accuracy when looking for changes in performance.

I’m extremely skeptical…in fact I don’t believe it at all.

The computer modeling and aerodynamics testing car manufacturers go through…they would have found that solution on day one.

It looks real good though.

Agreed, dubious MPG improvement claims. Drop an air dam off that puppy to channel air around the dirty underbody and around the front tires and then we’ll talk MPG improvement.

Today’s cars spend a great deal of time in wind tunnels during the design stage. Every tiny fraction of a MPG increase is incorporated since the CAFE standards rule the roost.

A Prius in particular has every bit of gas economy squeezed out of it.

So I think it would be a waste to second guess the designers.

In the bad old days when the stylist called the shots it would have been easy to add some items to make the car more aerodynamic.

The only way I can imagine this working is if it blocked airflow to a needed area of the car, like the radiator!

This guy is getting 95 mpg at 65 mph out of an aero-modded Honda Civic. An undertray (smooth panel on the underside of the car) is one of his mods. Don’t know how much of factor it is by itself, but all together the aero mods result in impressive efficiency gains.

The only way I can imagine this working is if it blocked airflow to a needed area of the car, like the radiator!
Define "needed."

The cooling requirements of a car, driving uphill, in Death Valley CA, in August…are drastically different from those required of a car driving a straight-and-level 55 in Duluth in February. A car that required frequent cowl adjustments would be unmarketable, so automakers plan for the “worst-case-scenario” and size the opening accordingly. If you have a high “PITA tolerance,” you could block off as much of the cowl as conditions permit and realize a gain. Aerodynamically, a cooling inlet is a giant bucket/parachute shape, so pretty much any rad block would be “better” than none.

(Proving my point, modern cars with computer controls DO adjust the cowl openings as you drive, for the purpose of MPG gain, showing that there are real aero gains hiding here. No reason, other than inconvenience, that this couldn’t be done low-tech, with pushrods and bellcranks, or even duct tape!)

@circuitsmith: Unless your friend is increasing MPG as a hobby, his gains (while real) will probably not be “worth it.” Here’s why:

A Prius gets 48 highway. At $3/gallon, that means it costs 6.25c/mi. If he can up his MPG 5% (which is optimistic, IMO)…he’ll now get 52 MPG with a cost of 5.95c/mi–a savings of 0.3c/mi! It would take as LONG time to recoup parts and labor at that rate. OTOH, a pickup that gets 18 MPG costs 16.7c/mi to drive and a similar 5% MPG jump (to just 19MPG, BTW) would save 0.8c/mi…paying back in around 1/3 the time. ALSO, a Prius is already aerodynamic, whereas a truck is a brick on wheels, meaning there’s far more “low hanging fruit” to get.

TL;DR aero gains are possible, but only “worth” pursuing on a gas hog (if at all).

I too am suspicious of that claimed 10% improvement, or anything close to it.

That article’s credibility dropped to zero for me when, at the end of the page, the author pitched his book on Amateur Car Aerodynamics - for only $48!


The author used a relatively large sample base, which can prove how repeatable his measurement procedure is. Repeatability is more relevant than absolute accuracy when looking for changes in performance.

Without a dynometer it is very difficult to get accurate (or repeatable) MPG measurements. Again, they are colored by the driver’s goals, perhaps unconsciously. If the driver makes some changes and expects the MPG to go up, it will go up!. Unconsciously, he will drive more conservatively, which boosts MPG. This has been proven many times, in the example I cited previously. Only a double blind set of experiments could eliminate that factor. That is why any reliable scientific study is always double blind.

meanjoe75fan wrote “Unless your friend is increasing MPG as a hobby…”

LOL, not only did I get to stir up all the skeptics here, I’ve made a new friend!

p.s., I’m also skeptical, but I appreciate his fabrication skills.

Count me in as skeptical as well. The real number may be .010 or even .0010 instead of 10%.

Oh boy is that some bad body work. I wouldn’t drive that car regardless of the mileage. Kinda looks like an old Renault? I mean Citroen I think.

I’m not really a skeptic. A car is a series of compromises; you can “tune” a car to be better at any one performance statistic (generally at the expense of all the other characteristics)…and it really isn’t that hard to do. For instance, if I wanted to increase skidpad over stock (and didn’t care about anything else), it’d be simple: super-sticky tires that only last 1,000 miles, a harsh suspension, and up goes skidpad (while making the car “worse” as a whole).

The same goes for max HP, off-road capability, etc: but when “tuning” for MPG comes up, certain people get their hackles up and say “it can’t be done.” Of course it can–that’s not the hard part. The hard part is increasing MPG without simultaneously making the car so inconvenient as to overwhelm the fuel savings. (For instance, that belly skirt might save $5 in fuel per oil change. I’d gladly accept $10 in costs simply to not have to mess with all the fasteners every oil change…so, technically possible, but not practical.)

The skirt doesn’t cover the oil drain; and he could put in a cutout to reach the oil filter.