The Recumbent Bicycle and Human Powered Vehicle Information Center

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. 

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. 

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”. 

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.

This article on the AutoSpeed site states 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%. Here are some
low rolling
resistance tires tested by Green Seal and a

report 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 from JC Whitney 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

here. 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. I
  like the Scangauge II. $159 and it just plugs into the OBD port of your
  car. It works on almost all cars newer than 1995. New is the
  PLX Kiwi MPG
  device for only $90, though they seem to always be on backorder.

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

Electrolite el-11, a 3 wheeled electrothon
vehicle built by E. Michael Lewis

The 2007 Aptera concept, by
Aptera (formerly
Accelerated Composites)
– Cd of 0.11

 

2000 GM Aptera 108MPG Concept Car – CD of
~0.15

1983 Ford Probe IV – .0152 cd

1985 Ford Probe V Concept Car – Cd of 0.137

High Mileage
Loremo 2007 Concept Car
– Cd of 0.20

Honda FCX Fuel Cell
2008 Concept

The 2008
FuelVapor Al
pre-production car

VW 1 Litre concept car – Cd of 0.159

2012 VW XL1 Diesel Hybrid concept – cd of
0.19

2013 Powercore Suncruiser solar racing car

Mercedes IAA concept with extendable tail
2015 – 0.19 cd

Immortus solar electric car 2015 “unlimited
range” 😉

Links

Vehicle Mileage as a
function of speed
General
Car Aerodynamics Principals
Mechanical
Engineering Smooth Body
EcoModder forum

EcoRenovator site