A Practical Guide to Aerodynamic Modification

Updated August 15, 2023

Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive.

Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home?

The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many people have modified the aerodynamics of their cars to make them more efficient, go faster, handle better, or feel more stable. Here’s how.

Read, read, read

First you need to get a firm grasp on the basics of fluid dynamics, car aerodynamics, and techniques for testing and measuring modifications. Good sources include textbooks, such as Aerodynamics of Road Vehicles, published by SAE and now on its fifth edition. It’s expensive but well worth the money if you have any interest in aerodynamics, and at 1200 pages it is the single most comprehensive source available. SAE papers are a great resource too; when you read them, pay attention to the process OEMs use to develop new models: what changes did they test? How did they test them? How did they set program and design goals?

There aren’t nearly as many books on aerodynamic modification of existing cars, but you can still find good ones. Modifying the Aerodynamics of Your Road Car and Car Aerodynamic Testing for Road and Track, both by Julian Edgar, are fantastic resources. They contain a mountain of information on test techniques you can use to measure things like drag and lift on your own car, including tuft testing, pressure measurement, and throttle-stop drag measurement. Update: Edgar's newest book, Vehicle Aerodynamics: Testing, Modification & Development, came out the summer of 2023 and is available now. I can't recommend it enough; it is easily the most thorough reference for anyone interested in modifying their own car or truck.

While online resources such as forums might seem like a good place to start, they really aren’t. Published literature is usually trustworthy; books and articles have been peer-reviewed or have technical consultants who check for accuracy, misinformation, and misrepresentation. Online commenters, even those who seem like they know a lot, are a complete unknown. They might be a retired or practicing engineer who knows what they’re talking about, sure, but they might also be a teenager in their parents’ basement who read a blog post once. How would you ever know? Don’t rely on forums for information on aerodynamics; they spread misinformation like wildfire. That’s not to say that there aren’t commenters out there who are worth listening to, but it is very difficult to sift through all the nonsense to find them.

Set a Goal

Okay, you’ve read some books and are ready to start modifying your car. Now what?

Time to set a goal. Don’t confuse goals with dreams. Dreams are vague: “I want to improve the fuel economy of my car by reducing its drag.” Okay—improve its economy by how much? Reduce its drag by how much? How will you know you have accomplished your dream? Goals are concrete; they are measurable. “I want to reduce the aerodynamic drag of my car by 10%.”

Goals should also be achievable. If you set a goal that is completely unrealistic, you’re just setting yourself up for failure. Cutting the drag of a car in half, say, is something that’s likely attainable only through tremendous effort if at all. Deciding to increase downforce by 1500 lbf isn’t going to happen. Set a more realistic goal to start with, something you think you can accomplish. If that turns out to be too easy, you can always set another.

How should we pick a target? This is a bit harder to specify; it really depends on the car, on the person modifying it, and on the resources available to them. Reading the literature—textbooks, SAE articles, books on aerodynamic design and modification—can help you get a sense of what sort of change or improvement is possible on your car and should inform your goals.

For example, I recently acquired an older Toyota pickup. Given its high aerodynamic drag, having read a lot about the typical range of improvement possible with add-ons like air dams and spoilers on truck shapes, and looking at the differences in body design and details between this 30-year-old truck and modern pickups, I decided to set a goal of reducing its drag by 20%. More than that is probably possible, but I also decided I did not want to impact the functionality of the truck too much, by keeping minimum approach and departure angles (I decided on these by looking at typical angles of trucks on the market now) and retaining things like a usable open bed. Other considerations that might influence your goal can include ease of maintenance, looks or style, impact on resale or value, buildability, and cost—of both time and materials. Taking all those into consideration, set a goal that is realistic and achievable but gives you something to shoot for; I think 20% is attainable but I know I won’t be able to do it simply by removing the mirrors. This gives me something to aim for that I can probably accomplish but will still be a challenge.

Yes, it's got a long way to go.

Goals should be specific. If they are vague or open-ended you won’t have any way of knowing if you achieved them or not. “I want to reduce the drag of my car” is not a fantastic goal. “I want to reduce the drag of my car by 10%, keep it looking close to stock, and be able to use my trunk” is.

Make a Plan

Plan out how you will achieve your goal by laying out your objectives. Objectives are short-term steps that will lead to accomplishing the goal. For example, if I set a goal of running a 5K, my objectives might be to run certain distances in the weeks leading up to it or to complete a certain amount of cumulative mileage or exercise a certain number of days per week. If I set a goal, as in the example above, of reducing the aerodynamic drag of my truck by 20%, my objectives will outline the intermediate processes that will get me there.

Objectives will encompass all the various paths and projects you will have to take on. First up for me is to build and test a throttle-stop so I can actually measure changes in drag. Once that’s done and working, I’ve outlined 6 or 7 devices I want to test—from mirror removal to grill blocking to a full air dam to a tailgate spoiler—that will give me a better idea what changes lead to what percentage drag reduction. Then, my next objective will be to lay out a plan for which changes I want to make permanent, followed by the actual building and installation of them. Meeting each objective in turn will lead to accomplishing my goal. If halfway through it looks like they won’t get me there, I can always adjust my goal or try another route to get the truck where I want it to be.

Tuft testing side window flow with and without the mirror and A-pillar vortex generators. The vortex generators didn’t do anything, so I won’t bother making permanent ones.

Test and Measure

This is the fun part! While setting a goal and laying out objectives are both conceptual, testing and measuring are very hands-on; you get to actually go out and try things in real life. This is an essential part of any aerodynamic modification process. The airflow over cars is very complex and hard to predict; you must test if you want to know what effect a change has, whether that is through CFD simulation, in a wind tunnel, or on the road. This is the foundation of aerodynamic engineering, and it is why manufacturers spend so much money on wind tunnels and computers.

It’s a good idea to test not just single designs of one part but a range of parameters. Don’t just build a cardboard air dam at one height; try cutting it progressively shorter during testing to see if there’s an optimum height. Don’t put a wing on your car and test it at only one angle; try a range of angles while measuring lift and drag. This will tell you about the behavior of devices on your car, which is invaluable information. For instance, if that air dam reduces lift the most at its medium height but drag the most at its lowest height, you can use this information in the future if your goals change. If you’re modifying your car for track days, knowing the lift and drag of the wing at various angles on your car will let you optimize your setup for a particular track.

Measuring panel pressures behind an external duct to see how much they changed.

Assess

Evaluate the results of measurements and compare them with your objectives and overall goal. Deal with setbacks; use them as opportunities (“now I know this doesn’t work and am better off for it” rather than “that didn’t work, I’m a failure”). Build on successes (device A worked; if I modify it this way, will it improve?) and setbacks (device B didn’t work; if I modify it this way, will it improve?). Spreadsheets are your friend; plot the results of drag or lift or surface pressure compared to parameters of a shape change. Sometimes it’s easier to see patterns when information is displayed visually rather than in lists of numbers.

Then, modify your objectives or goal if necessary. “Hmm, this air dam design isn’t reducing drag as much as I had hoped; what if I combine it with a grill block or turning vanes?” or, “I achieved my goal too easily; I bet I can do more—what about aiming at a 15% drag reduction instead of just 10%?” Don’t be afraid to scale back your goal too; if it turns out your initial expectations were too high, that’s just fine, it’s your project. At the same time, don’t forget the adage “necessity is the mother of invention.” You’ll get more satisfaction out of a project to improve your car if you know that you did everything possible to succeed.

Don’ts

Things to avoid:

• Don’t follow a “template.” Streamlined profiles suggested as templates—often based on “teardrop” shapes or bodies of revolution—are arbitrary and pretty much useless for modifying a real car, especially in the ways many online forum users suggest. They don’t take into account what the airflow actually does over your car, which is tremendously influenced by its particular shape and details. They simplify a shape into a two-dimensional profile when the flow over cars is highly three-dimensional. They rely on unrealistic descriptions and predictions of fluid flows to explain. They’re fine as idealized shapes but not really applicable to actual cars. Ask yourself: if one of these templates were “perfect” for a car shape, why don’t all cars--even/especially very low-drag cars--look like them?
• Don’t guess and hope. Lots of people build something permanent without testing and then claim that it works because it “should.” Or they build something and perhaps find that long-term fuel economy has improved but have no idea if that was the best design because they never tested anything else. If an air dam actually had its lowest drag at one height but I built it lower or higher because I never tested it and so don’t know that, then I’ve left that potential on the table and am worse off for it for no reason other than my own ignorance. Nihil dulcius quam omnia scire (“nothing is sweeter than to know all things”).

Did this tail reduce drag or was it all a waste of effort? I don't know; I never tested it.

• Don’t take advice or claims at face value. Many people, especially online, are more than happy to chime in with what they think the results of a particular aerodynamic change will be, or blather on about what someone should do to their car. These people almost invariably don’t know what they’re talking about, and outlandish claims about fuel economy or drag coefficients almost always fall apart under scrutiny (like the Aerocivic example above--the drag coefficient its owner claimed was deduced from coast down testing, which is notoriously unreliable, and its actual long-term gas mileage fell far short of its owner's claims).
• Along with that, don’t rely on untrustworthy or incredible sources. Why are you taking Aerogeek2103’s advice to build an enormous tail on your car, following their template profile (see above)? The f*** do they know? Peer-reviewed literature exists for a reason; use it.
• Don’t fall into the trap of believing that aerodynamic modification is easy! There are scads and scads of commenters on forums all over the Internet who are convinced that car aerodynamics can be understood through a few simple principles. This could not be further from the truth. Just because you remember learning about the Bernoulli equation in high school physics and have some images of NACA airfoils saved to your hard drive doesn’t mean you now have the knowledge to recommend aerodynamic modifications and predict exactly how a given shape will perform.

Take it from me, someone who has fallen into each of these traps at some point over my journey through the world of aerodynamic modification (some multiple times): avoid them at all costs. They are nothing more than a waste of time.

You Have the Power

Modifying your car can be an extremely satisfying experience and modifying its aerodynamics especially so. When you measure something and find that it works the way you wanted it to, it can feel like magic, like you’re in control of the force exerted by this invisible fluid as it rushes around and over your car. Using this control to make your car handle better, go faster, feel more stable, or use less fuel offers tangible benefits if you’re willing to put in the work of reading, planning, testing, and evaluating.