Car Aerodynamics 101

Passenger Car Aerodynamics
Getting the most out of the car you have.

By Warren Beauchamp - Updated 5/2009

Most of the information about car aerodynamics seems to be centered around generating downforce. While this may be needed for race cars, the average 3000+ pound car driving at speeds below 90 MPH does not need to be concerned with downforce. If you are trying to improve the efficiency of your vehicle, reducing the coefficient of drag (Cd) should be the main concern.

Rationale
In this day and age of expensive fuel and inefficient vehicles, it makes sense both economically and ecologically to conserve as much fuel as possible. To accomplish this, you could go out and buy another car with better mileage, but there are other options. This article focuses on how to optimize your current vehicle.

The example vehicle is a 1998 Nissan Maxima. This is a rather boxy 4 door sedan with quite a lot of ground clearance and a 190hp 6 cyl engine, that is rated at 26MPG highway by fueleconomy.gov, but gets around 21MPG in mixed driving. This car in stock trim has a Cd of 0.32.

1998 Maxima Before mods

For highway driving conditions, it is estimated that driveline uses about 15% of the total energy to required to push your vehicle down the highway, tire rolling resistance represents about 25%, and air drag is about 60%! While the traditional sources advocate saving fuel by driving less or driving slower, there are greater gains that can be made by modifying the aerodynamics, engine, and rolling resistance of the vehicle. These modifications are not without cost, but are within reach of even those of us with meager incomes. All of the aerodynamic modifications mentioned here can be performed for under $1000, providing you are willing to do the work yourself.

It may take a couple of years for the dollars expended in making the modifications to be paid for by the savings of gas, but a payback in that timeframe is easy to rationalize to yourself, and others.

Vehicle	Configuration	MPG	Gas cost/year	Savings/Year
6cyl sedan	stock	26	$1615	$0
4cyl econobox	stock	40	$1050	$565
4Cyl hybrid	stock	50	$840	$755
6cyl sedan	aero mods	34.5	$1215	$400
Savings using the 6cyl sedan as "baseline", and using gas costs of $2.80/gal and 15,000 miles/year

As seen in the table above, purchasing a 4cyl econobox or a 4cyl hybrid to replace your comfy (and paid for!) 6cyl sedan would save a bunch of money every year, but not enough to pay for the replacement. If you can afford it, it does make the best sense from an environmental point of view, but purchasing an expensive new car just to save $900 per year in gas is not an option many of us can afford. To most of us it makes more sense economically to keep driving our current gas guzzler. Modifying the sedan to get 25% better mileage, for under $1000 would start paying back after only two years. None of the modifications below in itself will provide a huge change in efficiency, but 3% here and 5% there all add up to big numbers eventually.

The 25% mileage improvement figure above is an estimate based on results I have seen of a 70 MPG Honda Civic (Bryant Tucker), and a 32 MPG truck, (Phil Know). This would be an improvement in highway mileage only. The $1000 project cost estimate would be spent on:

Eibach height adjustable springs - ~$300.
Aluminum sheet and hardware to build a belly pan and other aero mods - ~$300
The remainder would be for other stuff like measuring the mileage.

Manufacturers design most cars for looks, with aerodynamics as an afterthought. As such, much can be gained by tweaking the aerodynamics of these vehicles. The unit of measurement for aerodynamics is called the "coefficient of drag" or Cd. The Cd value tells us how efficiently the vehicle slips through the wind. Another common measurement multiplies the Cd times the total frontal area of the vehicle. This is called CdA. Check this site for the Cd value for different cars. Lower Cd means better Mileage!

Here are things that can be done to improve your vehicle's aerodynamics:

Lower the car - Lowering the car reduces the effective frontal area, increasing efficiency. Note that this only works up to a certain point. There will be an ideal ride height for each car. According to this article, 2.7" ground clearance is a good minimum height to shoot for. According to Mercedes, "Lowering the ride height at speed results in a 3-percent improvement in drag."
Remove that wing - Many "sports" cars have a non-functional wing on the back. Removing it will improve the fuel economy. The exceptions are the small rear fairings that are designed to detach the airflow from a rounded trunk.
Clean up the underside of the car. - Installation of a "body pan", while a labor intensive operation, will provide a significant improvement in mileage. More...
If a body pan is not practical, an air dam will redirect air that would normally pile up under the car causing drag. Not as good as a body pan, but better than nothing. Should be combined with side fairings.
Fair the wheel wells. - Yeah, this looks funny, but completely covering the rear wheel well will help improve efficiency. While the front wheel can not easily be completely faired due to clearances needed for turning, a partial fairing can be made. In addition, fairings can be added in front and behind the tires to help transition the air around these large appendages.
Clean up the front of the car. Basically the smoother the better. If the car has a large air intake under the bumper, it may not need that opening above the bumper (they are often just styling cues). An aerodynamic plastic, composite, or foam and duct tape panel can be built to cover the opening.
Remove the side view mirrors and instead use a remote camera system.
Replace large whip antennas with smaller powered antennas.
Vehicles with steep windshields can benefit from a hood fairing to help smooth the transition of air between the hood and windshield.
A small "tail cone" can be affixed the the rear bumper to help transition the air from under the car.
Side fairings can be used to clean up the lower half of the body between the tires. More...

1998 Maxima after proposed modifications. Hover mouse over body mods to see notes.

Additional mods for trucks:
If you need the utility of a truck, there are things that can be done to improve their efficiency in addition to the items noted above. Most notably, cover the bed! A flat hard cover will help some, but a custom aero cover is much more efficient. Experimentation has shown that simple removal of the truck bed door does not provide better mileage.

Additional mods for Vans and SUVs::
A new spoiler design has been shown to reduce drag and lift significantly on bluff-backed vehicles such as minivans and SUVs. Simulations showed that aerodynamic drag on a mini-van moving at 67 mph were reduced by 5% when the new spoiler was attached. This rear spoiler acts like a diffuser when it is attached to the back of a vehicle, making the pressure on the back of the vehicle higher than without it. That's a good thing!
Full technical paper

Body Pans:
A body pan fairs the underside of the vehicle. This becomes increasingly important as the vehicle gets closer to the ground. The pan ideally covers the entire underside of the car, but this may be impractical in many cases, so the idea is to make it as smooth as possible. Covering the exhaust system can lead to heat buildup between the belly pan and the floorboards. In general it's a good idea to create a heat shield/tunnel extending from the engine compartment to the rear of the vehicle. This will serve to seal in as much of the heat as possible. High pressure from the engine compartment will force air down the tunnel and out the rear of the car. Also, louvers may be cut into the body pan in areas where more heat needs to be released, such as along the route of the exhaust pipe. NACA ducts do not work well for this application as they are designed as devices to scavenge incoming air without disturbing the airflow, not as an air exhaust device. Engine airflow needs to be retained, but generally there are large enough opening between the engine compartment and the front wheels to give good engine airflow, even with the underside of the engine covered.

Toyota Prius Body Pan
Be sure to make the areas where maintenance will occur easily accessible, especially oil pan drain and oil filter access. The belly pan should be parallel to the ground until just past the rear axle, then it should gradually curve upward to meet with the underside of the rear fascia of the car.
Even the most aerodynamic cars manufactured today, for example the Toyota Prius pictured here which is touted as having a full body pan, can be cleaned up extensively.

Car side fairings - "ground effects":
Most car bodies slope inward at the sides until they are inside of the tires toward the bottom of the vehicle, leaving a large gap between the tires. Mud flaps are spiffy but only serve to make the gaps bigger. This all adds up to a lot of aerodynamic inefficiency. Side fairings "fill the gap", transition the air around the tires and keep side winds from flowing under the car. If you are driving 60 MPH with a 20MPH side wind, 33% of the wind forces are on the side of the car, so making the side of the car aerodynamic is also important. Stylists have created "ground effects" that claim to be aerodynamic, but really aren't. Instead, a flat panel slightly wider than the tires can be installed to help fair the sides of the car. Check out the side of NASCAR vehicles for reference. This panel should extend down to meet with the body pan. The corner where the two panels meet should be rounded if possible. The hardest part of this task will be the door cutouts and clearances. Side fairings also transition the air around those large appendages called tires.

Turbulators, etc:
In areas where the body transitions at a rate of more than 12 degrees, turbulator strips, vortex generators, diffusers, very short fairings or other devices can be used to "trip the airflow".

The idea is that areas like the transition between the roof and rear window on the average car creates a large vortex. Any large vortices effectively grab the car and try to hold it back as it tries to slip through the air. If the air that makes up the vortex can be "tripped" before it leaves the back of the car, it will make smaller vortices, which will have a smaller effect on the overall aerodynamics of the vehicle. Measurement of the effects of these devices at highway speeds has been difficult to obtain.

Vortex generator above a Mitsubishi rear window
(photo by Mitsubishi)

Tire Fairing Flaps:
Many newer cars have small flaps in front of each tire. These flaps are designed to creat turbulence which helps to deflect the air around the tire and suspension components. An older article on the AutoSpeed site stated that about one-third of the total drag in an already aero car is caused by undercar flows, with most of that from the front wheels. Care must be taken to not make the flaps too big, as that can increase the drag instead of reducing it.

Tires:
Tire rolling resistance (RR) also plays a large part in the mileage of a vehicle. Running your tire pressure at higher pressures will help somewhat (do not exceed rated pressures printed on the side of the tire), but specially designed low RR tires will help more. The typical 20% reduction in RR from a low RR tire can result in fuel savings of 2% to 4%. Low RR tires are now commonly available. Here's a report on low RR tires by the US government. Green Seal notes that a typical Ford focus can increase it's mileage by 2 MPG (from 30 to 32MPG) just by replacing the stock tires with low RR tires. A caveat however, is that low RR tires do not handle as well as normal "sport" tires.

Wheel covers:
Unfortunately, the coolest looking chrome spoked wheels are really bad aerodynamically. The best wheel cover is a slightly convex, completely smooth cover that fits flush with the tire. "Racing disks" like the one pictured here or something similar can be snapped onto most wheels for a quick aero fix.

Temperature
Air temperature has a large effect on gas mileage. Part of this is due to rolling resistance. Because tires lose one PSI for every 10 degrees, and tires lose elasticity in colder weather, rolling resistance increases as temperature decreases. This means the tires don't roll as well when it's cold out. Air density also increases as temperature drops. Ralph Kenyon worked out the math to calculate how much this effects gas mileage. His works suggests that gas mileage drops 2% for every 10 degrees F below 90 degrees due to air density alone. This means that at 40 degrees F there will be a 10% decrease in mileage.

Engine efficiency:
Modern engines are fairly efficient. Plenty of claims for products to improve your vehicles engine efficiency have been made, but few do anything worthwhile. The ones that do work are generally pricey. If you want to spend the bucks, you can:

Install headers or a "Y pipe" to scavenge the exhaust gasses. Do not remove the catalytic converter.
Install efficient mufflers. Note that engines do require backpressure to function properly.
Install Under-drive pulley. Note that this will reduce engine cooling and and battery recharging. Most vehicles are designed for worst case scenarios though, so this is usually ok unless you have a 3 kilowatt stereo.
Install a cold air intake. Most air intake systems are designed to be quiet, not efficient.
Install a high flow air filter.
If the radiator fan is driven off of the engine by belts, replace it with thermostatically controlled electric fans.
Install a transmission with taller gears. Once you have made your vehicle more aero, it won't need the power that the extra RPMs provided. Taller gears mean that the engine RPMs will be lower, which equates to less gas used.

Note that due to differences in how engines operate, changing the intake or exhaust system may not help the mileage. Generally they don't hurt it, but you may get lower mileage due to the tendency to drive more aggressively when you can hear the engine making cool noises. Measuring is key.

Measuring your mileage:
So, you have decided to terrorize your car, and are not too concerned about what your neighbors will think. Now, how do you figure out if what you did helps or hurts your mileage? You have a couple choices.

Record the amount of gas and your mileage and do the math. Here's how:
1) Fill up your car. Record the mileage.
2) Next time you fill up, record the mileage and the amount of gas.
3) Latest mileage minus original mileage = number of miles driven
4) Number of miles driven divided by amount of gas = miles per gallon
This is the cheapest thing to do, but takes a long time and is not very granular.
Buy a mileage measurement device. One option is Scangauge which just plugs into the OBD port of your car. It works on almost all cars newer than 1995. Another option is the PLX Kiwi MPG device.

Actual Measurements and Modifications - Warren modifies the Maxima

The Future
In the near future, Joe Sixpack will become more comfortable with the look of aerodynamic vehicles. As the model below crafted by Raymond Gage shows, aerodynamic vehicles can be quite stylish.

While this vehicle is only a concept today, economic and ecological pressures will combine in the near future to force vehicle manufacturers to build true "No Compromise" aerodynamic vehicles. Below are some more nice shapes.

Oldsmobile Aerotech concept car