How accurate are gas mileage monitors?

Ever since the meteoric rise of gas prices in 2008, gas
mileage has become a top priority. The focus turned not only
to the Environmental Protection Agency-rated mileage of our
cars, but also to how that mileage could be optimized. It was
even an issue in the 2008 U.S. presidential race, when
then-candidate Barack Obama suggested that Americans check
their tires, since proper inflation can dramatically improve
gas mileage.

There's a problem with calculating accurate gas mileage,
though, and there always has been. Most of us do a simple
calculation each time our gas tank hits empty: number of
miles on our odometer divided by the number of gallons we can
fit in our gas tank. In theory, this provides an accurate
assessment of how many miles we can drive per gallon. In
reality, the number we come up with is a mere estimate that
fluctuates by the tank, and even by the day.

This is perfectly fine for coming up with a rough number, but
things start to get hairy when you start trying to improve
your gas mileage. How do you know if the techniques or
gadgets you're using are working if a rough estimate is all
you can get? If your car's mileage varies by about 10 percent
based on driving conditions, how do you know if the
improvement you see after, say, adding acetone to your gas
tank is a result of the additive or of the weather?

Lots of factors affect your car's mileage. For instance,
snowy weather requires headlights during the day, wipers and
defrosting your front and rear windshields. All this activity
uses gas. On the other hand, warm weather might mean
activating the air conditioning, which also lowers your
mileage. And then there's the fact that gasoline takes up
less volume in the cold and expands in the heat, so the
amount of gas you can fit in your tank isn't even constant.

Weather, hills, road conditions and frequency of stoplights
all affect how many miles you get per gallon at any given
time, so manual calculation after you've gone through
an entire tank doesn't do much for determining an increase or
decrease in MPG. The solution to this problem is one that
briefly popped up in the 1970s due to the gas crisis and then
promptly lost favor when prices went back down. It's
real-time energy feedback. The concept made a huge comeback
when gas prices start averaging $4 a gallon.

We'll look at several approaches to instantaneous MPG
feedback and find out if, and how, they work to improve your
car's efficiency.

Gas Mileage: The Great Gas Quest

New gas-saving gimmicks are popping up all the time these
days. Adding acetone, aka nail-polish remover, has been
touted to increase mileage. It doesn't. Same goes for putting
a magnet on your fuel line and installing a water-injector
system in your engine. Water injection really only increases
the efficiency of a turbocharged or supercharged engine.

Our driving styles, on the other hand, have a surprisingly
acute effect on how many miles we get out of a gallon of gas.
For instance, accelerate too quickly and mileage decreases,
because the injector has to squirt more gasoline into the
engine to achieve a speed quickly than to achieve it slowly.

Incidentally, accelerating too slowly is also bad for MPG,
since it keeps your car in a lower gear for too long. Lower
gears are less efficient than higher gears.

Other factors reducing gas mileage include sudden stops and
high speeds.

Most of us are aware of these factors, but making
a long-term change in how we drive requires more than
awareness. It requires a lot of attention. And we're usually
too busy with getting where we're going to remember to start
and stop more gradually. Enter the real-time MPG monitor.

You find these on lots of cars, including many hybrids and
even a few nonhybrids. They come in a range of forms. BMWs
have had a low-tech version of the energy gauge for many
years, in the form of a vacuum gauge situated in the
instrument panel. This simple device keeps track of the level
of vacuum in the intake manifold, which moves gasoline and
air from the carburetor to the engine's intake valves. The
vacuum is highest when you're cruising in high gear and
lowest when you're idling. The greater the manifold vacuum,
the greater your gas mileage. A vacuum gauge gives constant
feedback on manifold vacuum, so you know in real time exactly
how your driving is affecting your fuel efficiency.

A much more high-tech approac­h to real-time monitoring has
popped up recently, most famously in the Prius. The Prius
Energy Monitor is an LCD screen immediately to the right of
the driver that keeps track of some great data, including
whether the engine is drawing power from gas or from the
battery and, of course, the car's MPG at any given moment.

But are these gauges accurate? And do they even need to be?