If you're like me, you like to squeeze every last mile you
can out of your tank of fuel. If you could get 20 miles extra
from each tank, that could save you two or three trips to the
gas station over the course of a year.
main impediment to stretching your mileage is the fuel gauge
on your car, which makes you think you have less fuel than you
actually do. These devices are notoriously inaccurate, showing
empty when there are gallons left in the tank and showing full
for the first 50 miles.
In this edition of HowStuffWorks,
we'll learn why our fuel gauges behave the way they do. There
are two main parts to a fuel gauge: the sender, which measures
the level of fuel in the tank, and the gauge, which displays
that level to the driver. First, let's see how a typical
The Sending Unit The sending unit is
located in the fuel tank of the car. It consists of a
float, usually made of foam, connected to a thin,
metal rod. The end of the rod is mounted to a variable
resistor. A resistor is an electrical device that
resists the flow of electricity. The more resistance there is,
the less current will flow. In a fuel tank, the variable
resistor consists of a strip of resistive material connected
on one side to the ground. A wiper connected to the
gauge slides along this strip of material, conducting the
current from the gauge to the resistor. If the wiper is close
to the grounded side of the strip, there is less resistive
material in the path of the current, so the resistance is
small. If the wiper is at the other end of the strip, there is
more resistive material in the current's path, so the
resistance is large.
In the sending unit, the fuel
has to drop below a certain level before the float starts to
When the float is near the top of the tank, the wiper on
the variable resistor rests close to the grounded (negative)
side, which means that the resistance is small and a
relatively large amount of current passes through the sending
unit back to the fuel gauge. As the level in the tank drops,
the float sinks, the wiper moves, the resistance increases and
the amount of current sent back to the gauge decreases.
This mechanism is one reason for the inaccuracy of fuel
gauges. You may have noticed how your gauge tends to stay on
full for quite a while after filling up. When your tank is
full, the float is at its maximum raised position -- its
upward movement is limited either by the rod it's connected to
or by the top of the tank. This means that the float is
submerged, and it won't start to sink until the fuel level
drops to almost the bottom of the float. The needle on the
gauge won't start to move until the float starts to sink.
You may be surprised
at how much fuel you actually have left when the needle
is on empty. To find out, check your owner's manual for
the exact volume of your fuel tank. Then, the next time
your needle shows empty, find the nearest gas
station and fill 'er up. Subtract the number of
gallons it takes to fill your tank from the volume
stated in the owner's manual, and you'll know just how
much farther you can go when the gauge hits
similar can happen when the float nears the bottom of the
tank. Often, the range of motion does not extend to the very
bottom, so the float can reach the bottom of its travel while
there is still fuel in the tank. This is why, on most cars,
the needle goes below empty and eventually stops moving while
there is still gas left in the tank.
Another possible cause of inaccuracy is the shape of the
fuel tanks. Fuel tanks on cars today are made from plastic,
molded to fit into very tight spaces on the cars. Often, the
tank may be shaped to fit around pieces of the car body or
frame. This means that when the float reaches the halfway
point on the tank, there may be more or less than half of the
fuel left in the tank, depending on its shape.
Now let's see how the gauge works.
The Gauge The gauge is also a simple
device. The current from the sender passes through a resistor
that either wraps around or is located near a bimetallic
strip. The bimetallic strip is hooked up to the needle of the
gauge through a linkage.
As resistance increases, less
current passes through the heating coil, so the bimetallic
strip cools. As the strip cools, it straightens out, pulling
the gauge from full to empty.
The bimetallic strip is a piece of metal made by laminating
two different types of metal together. The metals that make up
the strip expand and contract when they are heated or cooled.
Each type of metal has its own particular rate of expansion.
The two metals that make up the strip are chosen so that the
rates of expansion and contraction are different.
When the strip is heated, one metal expands less than the
other, so the strip curves, with the metal that expands more
on the outside. This bending action is what moves the needle.
Some newer cars, instead of sending the current directly to
the gauge, use a microprocessor
that reads the output of the resistor and communicates with
the dashboard. These systems actually help improve the
accuracy of the gauge. Let's take a look at one of these
Microprocessor-controlled Gauges Some newer
cars have a microprocessor
that reads the variable resistor in the tank and communicates
that reading to another microprocessor in the dashboard.
Carmakers can tinker with the gauge movement a little -- they
can compensate for the shape of the tank by comparing
the float position to a calibration curve. This curve
correlates the position of the float with the volume of fuel
left in the tank. This allows the gauge to read more
accurately, especially in cars with complicated gas-tank
Systems like this can also trigger a fuel light that
signals when fuel is getting low. Most of these lights come on
while there are still a couple of gallons of gas left in the
tank, giving you plenty of time to stop for fuel.
The microprocessor can also provide some damping to
the needle movement. When you go around a turn, or up a hill,
the fuel can slosh to one side of the tank and quickly change
the float position. If the needle were to respond quickly to
all of these changes, it would be bouncing all over the place.
Instead, software calculates a moving average of the last
several readings of the float position. This means that
changes in needle position occur more slowly. You may have
noticed this when filling up your car -- you'll finish filling
the tank long before the needle reaches full.
While fuel gauges are far from exact, they err on the
For more information on fuel gauges and related topics,
check out the links on the next page.