Commentary: How to stretch that $3.50-a-gallon tank
J.J. Zhang is a chemical engineer and
amateur financial adviser who was the winner in MarketWatch’s second annual
World’s Next Great Investing Columnist contest. He runs the blog MarketTech Reports. You can
follow him on Twitter @MarketTechRpts.
Taking a break from normal topics, let’s talk about something more near and
dear to your wallet: gas prices. Though gas prices have climbed, it’s still
amazing how cheap gasoline is in the U.S. compared with other countries or
products.
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To put it in perspective, a bottle of water at my local vending machine is
equivalent to $12 per gallon and milk goes for $3.40 per gallon. In comparison,
a $3.50 gallon of gasoline requires oil to be found, dug up, refined, shipped
all over the world and driven finally to your local gas location. That one
gallon allows you to travel 20-40 miles, equivalent to saving 10-15 hours of
walking.
Nonetheless, the U.S. economy and suburban lifestyle has developed in an
environment of cheap gas and thus rising prices at the pump have a big effect on
discretionary income. CNN estimated the average American household spent $368
monthly on gas in 2011 and AAA estimates the average American will spend over
$2,100 on gas in 2013.
Since it’s not always possible to switch to a more gas-efficient car or take
public transportation, here are seven tips to stretch that tank out as much as
possible.
1. Curb aggressive behavior
Perhaps the most influential on gas efficiency, but also the hardest to
change, is aggressive driving behavior. Frequent speeding, rapid acceleration
and deceleration all contribute significantly to lower gas mileage. This is
especially pronounced during highway driving where rapid stopping and rapid
acceleration will erode the mileage to levels comparable to city driving.
The more horsepower and car weight, the worse it becomes. Importantly,
aggressive starting and stopping can also cause traffic wave disturbances, a
common type of traffic jam.
The U.S. EPA estimates aggressive driving can cause fuel efficiency to fall
anywhere from 5% to 33%, which translates to $1 or more per gallon on the high
end. Less rapid acceleration and deceleration, and higher use of practices like
cruise control, really provide the best bang for the buck on fuel costs.
2. Cut the speed
Part of aggressive driving behavior, the speed being driven has a big impact
on a well-known physics problem: drag. The two sources of vehicular drag are
tire friction and aerodynamic drag. For aerodynamic drag, the two major
components are the drag coefficient, determined by the shape of your car, and
your velocity, squared.
Drag coefficients are generally fixed by the car structure. SUVs and trucks
have drag coefficients 50% or more above typical cars and tractor-trailers have
coefficients three to four times higher. This coefficient is also why
high-gas-efficiency cars such as Prius hybrids all have the same basic shape and
look; it’s the most optimal and practical shape.
As an example, the wind deflectors installed on tractor-trailers reduce drag
by almost 20%, a big cost savings for trucking companies. For cars however, very
small improvements can be made by removing unnecessary adornments from your car,
such as bike racks.
Because aero drag is proportional to the square of the velocity, going faster
becomes an exponential issue. Driving at 80 mph doubles the drag of going 60
mph, so driving slower on highways goes a long way to reducing costs. The EPA
estimates every 5 mph above 50 mph is equivalent to an additional $0.25 per
gallon.
3. Know when to use AC, when to roll down the windows
Fans of “Myth Busters” may have already learned this about the impact of air
conditioning vs. retracted windows. While it takes electrical power generated
from the motor to power the AC, this can compare favorably to the additional
drag caused if the windows are lowered. Generally at approximate highway speeds,
AC becomes more fuel efficient than lowering windows.
Related to this, on hot sunny days when the car interior turns blistering,
it’s more efficient to cool the interior initially by lowering the windows and
mixing the air with the outside rather than blasting the AC straight away.
4. Avoid idling
Many people assume gas is only consumed during actual driving and forget that
gas is used as long as the engine is on, such as when it’s idle. Due to how
combustion engines are designed, excluding some hybrid designs, there is a
considerable amount of gas consumed to keep the engine running.
It’s more efficient to turn off and restart than allowing even 10 seconds of
idling. Some research estimates two minutes of idling is equivalent to driving
one mile. The EPA estimates idle costs at approximately $0.01-$0.04 per minute.
The additional wear and tear from excessive restarting is potentially a
concern but may be minimal. Some European and Asian cars have a feature where
the engine is temporarily turned off automatically after a few seconds of
idling, such as during red lights.
5. Hold the weight down
Quite simply, the more you carry, the more energy and gas it takes to move
it. Smaller, lighter cars get much better mileage than larger cars, all else
being equal. For owners of more than one car, use smaller cars for
longer-distance trips. Even removing unnecessary items such as unused seats,
bike racks and stored items out of your car helps.
Fueling up to less than a full tank also helps slightly though this needs to
be balanced with the distance and frequency of refueling. The EPA estimates
every 100 pounds of weight hurt efficiency by 1%-2%, which translates to
approximately $40-$80 per year — small but noticeable.
6. Watch the ethanol blends
Many stations now sell gasoline blended with ethanol, a lower and less energy
dense fuel. E10, or 10% ethanol, results in about 3%-4% worse efficiency vs.
pure gasoline. However, E10 is generally now the standard fuel found in most
places and many states do not require labels.
E85, containing 85% ethanol, for use in flex fuel vehicles, does much worse,
exhibiting 25%-30% drops in efficiency. Keep this in mind when comparing prices
— cheap E85 gas may not be cheaper when all things are considered.
7. Mind the temperature at fill-up
I recently saw this passed around on the Internet. This concept takes
advantage of gasoline’s thermal expansion in combination with volumetric-based
gasoline pumps and underground stored-fuel tanks. By filling up in the morning,
when the ground is coldest, you get higher density gasoline than after the tanks
have warmed up over the course of the day.
Gasoline’s thermal expansion coefficient results in approximately 1% more
gas, by mass, for every 18 degrees difference between the daytime air
temperature and pumped underground cooled temperature.
Though clever, the true impact is hard to measure without accurate
temperature values but it’s unlikely to yield much benefit. In seasons or
locations where the ground temperature is comparable to the air, this effect
likely disappears.
(Credit: Lamborghini) 
The accident was reported around 6:40 a.m. Tuesday on Laguna Canyon Road between El Toro Road and State Route 73.
