Theory

How to Think About Car Aerodynamics
A very, very basic overview
Physical explanation, measurement devices, probes, and how to use them
Learn what yarn taped to your car can tell you about the airflow over it
A problematic method for measuring drag--with my suggestions for improvement
Coefficients in Aerodynamics Engineering
Just what is a "drag coefficient" anyway?
The difference between models and reality
Momentum loss in viscous flows
Things I Wish I Had Learned Sooner
Lessons from a Top-10 engineering degree program

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The Relationship Between Drag and Fuel Economy

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Lined up and ready to head out on track at the 2022 Green Grand Prix , an annual fuel economy competition held the Friday of opening weekend at Watkins Glen International. We all know that reducing drag can improve fuel economy. It’s intuitive but can be proven mathematically: when your car has less force acting against it, it takes less fuel (gas, diesel, or electric) to move. But what is the exact relationship between aero drag and economy? Can we measure it? Or predict it? Figuring this out can help us plan modifications to our cars with the aim of improving economy, as well as tell us if fuel economy is an accurate measure of changes in drag. ETA 5/18/2026:  Since writing this post, I've learned more about how to model fuel economy in cars. It comes down to efficiency (thermal, propulsive, and overall) the magnitude of resistance force, and the energy available in fuel sources like E10. See  this article on efficiency for a better explanation,  how to evaluate online ...
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How to Think About Car Aerodynamics: A Very, Very Basic Overview

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The study of aerodynamics is complicated. If anyone tries to tell you otherwise, run the other direction—it’s a sure sign they don’t know what they’re talking about.   Over the last two years especially, my thinking about aerodynamics and appreciation of its complexity has changed dramatically—a result of my going back to school to get another bachelor’s degree, this time in aerospace engineering where a good working knowledge of airflows is required and education in not just general fluid mechanics but also aircraft aerodynamic design forms a core part of the technical curriculum. I'm in the midst of my last semester now and to clarify my thinking at this point I decided to put some things in writing in the hopes they might help someone else as well as myself, specifically focused on car aerodynamics. A word of warning: I've tried to minimize the amount of math below, but some mathematical relations are unavoidable if you want to build an understanding of fluid flows. If anyth...
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How Spoilers Work

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ETA 2/28/2026: For a better explanation than this (which I wrote before enrolling in an aerospace engineering degree program), and better example of test methodology and data analysis, see my most recent post on spoiler testing. Buckle up, this one's long. After hanging around Ecomodder for years, I had a pretty messed up idea of the function of spoilers because of what I had read on that forum. You might too.   See, the prevailing theory there—based entirely on posts by one forum member who is widely seen as an aerodynamic “guru”—is that spoilers “reach out” to an ideal “template” profile . By so doing, the theory goes on, a “locked vortex” is captured and the airflow above the spoiler follows the “ideal” template line as the air just ahead of the spoiler recirculates. So, simply position a spoiler and extend it until it touches a template profile overlaid on a side-view image of your car and, voil à ! Lower drag—because the spoiler isolates negative pressure in front, higher pr...
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