We’ve all heard the phrase “As aerodynamic as a brick wall.” I certainly have, due in part to the fact that I drive a lifted SUV with all the off-road accouterments, each of which is as aerodynamic as a small, accouterment-sized brick wall.
I get a respectable 12 miles per gallon. Thirteen on the highway if I baby it. This got me thinking: Is my vehicle actually as bad as a brick wall? What would happen if I put an engine in a brick wall and drove it down the highway? What kind of fuel mileage would I be getting? Is this a practical and responsible thing to do?
A bit of technical explanation: Drag coefficient is a component of aerodynamic drag that is independent of vehicle size. So you can have a bus and a motorcycle that have the same drag coefficient, but the bus will have much more total aerodynamic drag because it is bigger. If you shrink ray’d the bus to have the same area (when viewed from the front) as the bike, they would then have the same total aerodynamic drag. If they had the same engine, they would have the same fuel mileage and the same top speed (assuming similar tires, air density, etc...). It would also be adorable. Imagine a tiny little bus taking chickens to work in the city. Oh no, the bus is running late and Meryl Cheep is going to be late to the egg factory!
What were we talking about? Oh yeah, aerodynamic drag. According to the book Fundamentals of Fluid Mechanics, by Alan Prasuhn, a square flat plane (like, say, a brick wall) has a drag coefficient of about 1.16.
Road cars are usually around 0.3, so 1.16 is pretty high. Are there any cars with drag coefficients that high? Actually, yes. Several race cars.
Cars that produce a lot of aerodynamic downforce also produce a lot of drag. A Formula 1 car’s drag is pretty high, especially at the slower tracks. According to Formula 1 Technical Analysis 2000, by Giorgio Piola, an F1 car in 2000 had a drag coefficient of just under 1.1 at Monaco, the slowest track. Not exactly a brick wall, but close enough for our rough calculations here.
Based on the fuel tank size and the distance covered between refueling, we can estimate that an F1 car at that time got somewhere around four miles per gallon. Since these cars have a similar drag coefficient as a brick wall, we can conclude that an F1 car-sized brick wall with an F1 V10 swap would get something like four miles per gallon on the racetrack.
Okay, this is a pretty extreme example; let’s go the other direction. What if we Prius-swapped our brick wall and hypermile’d it? What if we put a Toyota Prius drivetrain into a Toyota Prius sized brick wall and drove 50 mph in the fast lane, totally infuriating everyone behind us. What kind of fuel mileage are we getting then?
Running some calculations and assuming sea level air density, 0.01 tire rolling resistance coefficient, and the Prius’s impressive engine efficiency of 40%, a Prius will get somewhere in the ballpark of 70 mpg while driving at a constant 50 mph. A quick look at testing data posted by hypermiling geeks on the internet confirms this number. If we change the drag coefficient from the Prius’s 0.26 to our brick wall’s 1.16 and redo the calculations, we get a fuel mileage of (drum roll):
29 miles per gallon.
So there you have it. Your brick wall will get somewhere between four and 29 miles per gallon, depending on your engine and driving style. Not too bad. You can safely, and efficiently, engine swap your brick wall for road use.
Disclaimer: You cannot safely or efficiently engine swap a brick wall for road use.