Chapter 9 | 531use per vehicle in the United States has increased to 603 gallons in 1999
(worth $1206 at $2.00/gal) from 506 gallons in 1990 (Fig. 9–56).
Saving fuel is not limited to good driving habits. It also involves purchas-
ing the right car, using it responsibly, and maintaining it properly. A car does
not burn any fuel when it is not running, and thus a sure way to save fuel is
not to drive the car at all—but this is not the reason we buy a car. We can
reduce driving and thus fuel consumption by considering viable alternatives
such as living close to work and shopping areas, working at home, working
longer hours in fewer days, joining a car pool or starting one, using public
transportation, combining errands into a single trip and planning ahead,
avoiding rush hours and roads with heavy traffic and many traffic lights,and
simply walkingor bicyclinginstead of driving to nearby places, with the
added benefit of good health and physical fitness. Driving only when neces-
sary is the best way to save fuel, money, and the environment too.
Driving efficiently starts before buying a car, just like raising good chil-
dren starts before getting married. The buying decision made now will
affect the fuel consumption for many years. Under average driving condi-
tions, the owner of a 30-mpg vehicle will spend $400 less each year on fuel
than the owner of a 20-mpg vehicle (assuming a fuel cost of $2.00 per gal-
lon and 12,000 miles of driving per year). If the vehicle is owned for
5 years, the 30-mpg vehicle will save $2000 during this period (Fig. 9–57).
The fuel consumption of a car depends on many factors such as the type of
the vehicle, the weight, the transmission type, the size and efficiency of the
engine, and the accessories and the options installed.The most fuel-
efficient cars are aerodynamically designed compact cars with a small
engine, manual transmission, low frontal area (the height times the width of
the car), and bare essentials.
At highway speeds, most fuel is used to overcome aerodynamic drag or air
resistance to motion, which is the force needed to move the vehicle through
the air. This resistance force is proportional to the drag coefficient and the
frontal area. Therefore, for a given frontal area, a sleek-looking aerodynami-
cally designed vehicle with contoured lines that coincide with the stream-
lines of air flow has a smaller drag coefficient and thus better fuel economy
than a boxlike vehicle with sharp corners (Fig. 9–58). For the same overall
shape, a compact car has a smaller frontal area and thus better fuel economy
compared to a large car.
Moving around the extra weightrequires more fuel, and thus it hurts fuel
economy. Therefore, the lighter the vehicle, the more fuel-efficient it is. Also
as a general rule, the larger the engine is, the greater its rate of fuel con-
sumption is. So you can expect a car with a 1.8 L engine to be more fuel
efficient than one with a 3.0 L engine. For a given engine size,diesel engines
operate on much higher compression ratios than the gasoline engines, and
thus they are inherently more fuel-efficient. Manual transmissionsare usu-
ally more efficient than the automatic ones, but this is not always the case. A
car with automatic transmission generally uses 10 percent more fuel than a
car with manual transmission because of the losses associated with the
hydraulic connection between the engine and the transmission, and the added
weight. Transmissions with an overdrive gear(found in four-speed automatic
transmissions and five-speed manual transmissions) save fuel and reduce
FIGURE 9–56
The average car in the United States is
driven about 12,000 miles a year, uses
about 600 gallons of gasoline, worth
$1200 at $2.00/gal.30 MPG20 MPG$800/yr$1200/yrFIGURE 9–57
Under average driving conditions, the
owner of a 30-mpg vehicle spends
$400 less each year on gasoline than
the owner of a 20-mpg vehicle
(assuming $2.00/gal and
12,000 miles/yr).