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INTRODUCTION
Particularly for those just getting their Prius in the fall or winter
months this must be one of the first questions and most common questions
I've seen posted. I think I posted something similar myself when I first
got my Prius in late Oct. of 2003.
Toyota, rightly, advertises the Prius with the EPA determined gas mileage
of 51 highway, 55 combined, and 60 city. I think that as part of the
excitement of getting the car we all anticipate seeing those numbers
'right out of the box' when we pick up our car. More often than not that
doesn't happen. In this article I'm going to try to clarify the reasons
why. I'll also offer some suggestions on how mileage can be improved
toward the end.
THE EPA NUMBERS
When the EPA tests cars and rates their gas mileage they don't actually
drive on a road. They use a device called a dynometer. They use the exact
same pattern of driving that's been used for every tested vehicle for over
20 years. The "city" portion is nothing like what most of us consider city
driving...there are many stretches of 1/2-2 miles at 21mph. And although
there are many stops built in there is nothing like the stop and go
driving of a real city. Thus, many people in 'real' cities don't see
anything like the city mpg numbers.
The highway test portion is equally outdated with an average speed of
48mph. For those of us who use the interstate transit system in the US and
know that posted limits of 70mph are, in reality, minimum speeds with most
vehicles in the 75-80mph range this portion of the EPA test is equally
unrealistic.
To further exacerbate the issue of inaccuracy with 'real world driving'
the EPA tests all the cars in a temperature and climate controlled
building at 68-86 degrees Fahrenheit with a vehicle that is already warmed
up (highway only, city portion is cold start) and A/C turned off. It
assumes a flat level road (i.e. no hills). It measures exhaust CO2 to
determine how much fuel was used.
See How Vehicles Are Tested in detail
at this link, complete with photos.
a)Hybrids (as other cars) have their lowest MPG in the first 5-10 minutes
of driving while the vehicle warms up. Thus, for those of us with only a
5-10 mile commute we're unlikely to ever get the car into it's most
efficient operation condition.
b)Due to the battery power and the range of SOC of the battery,
particularly on the city portion of the test, much of the driving may be
under battery power and a small portion of energy used is never measured
by the EPA (as they have no means to do so) thus giving a slight higher
MPG rating than is really possible. They say that the battery SOC must be
at the same level at the beginning and end, but even so there is some
inaccuracy in that readout.
c)All cars perform less efficiently and require longer warm up times in
colder weather...even at 50 degrees there's an impact. This becomes most
noticeable in a high mpg vehicle like the Prius.
The best way to think about this is to consider the Prius compared to
another vehicle. If the Prius gets 55mpg in 72 degree weather but suffers
a 10% decrease in mileage at 45 degrees that means that your gas mileage
will drop to 49.5mpg--you're getting 5.5mpg LESS than you were at 72
degrees!! Pretty demoralizing. If you're driving a Camry that gets 25mpg
at 72 degrees and suffer that same 10% hit at 45 degrees you drop only to
22.5mpg...hardly even noticeable at a fill up since it's only 2.5mpg less
than before. It's the same 10% but it's relatively more due to the lower 'gallonage'
of the Prius (that is uses less gas for distance traveled).
OTHER FACTORS
Especially early in the Prius ownership there are a number of other
factors that come into play. Many of which will work themselves out over
time.
1)Break in--almost universally people notice an improvement in
mileage at about the 5000-6000 mile mark. The first generation Prius
owners and a few 2nd generation Prius owners who've passed the 20k mile
mark also note a 'secondary break-in' period. It seems that the engine
friction, wheel bearings and other moving parts 'loosen up' over that
first 5k miles. Also, the High Voltage Nickel Metal Hydride (NiMH or HV)
battery becomes more efficient over time.
2)Technique/style/learning curve--I firmly believe that the Prius
driver intuitively or through trial and error improves his/her driving
technique over time better take advantage of the most efficient modes of
driving their car. Not all drivers do this, but it becomes almost a game.
With time one learns to accelerate the vehicle in the most efficient
manner. He/she learns to anticipate stops and begin coasting or gliding to
those stops without wasting power to get there or putting undue stress on
the friction brakes.
3)Warmer temperatures--all praise to summer. The Prius is at it's
most efficient in warm weather...particularly in that range of
temperatures where A/C use is not necessary but where air temperatures are
in a range that is 'comfortable' to the car and HV battery. Almost
everyone sees a dramatic improvement in fuel economy in those summer
months and reports of exceeding the EPA numbers begin to frequently
appear on the forums!
4)A/C use--there is a significant impact on mileage with A/C use.
Unfortunately Toyota did not make it easy or intuitive to learn how to get
to the 'vent' mode to allow circulation of outside air without engaging
the actual heating/cooling system. In cold weather I either don't use the
AC at all since my car stays in the garage at home and usually remains
warm enough (with my coat on) that I don't have to use it, or I turn it on
'Auto A/C' at 65 degrees--the lowest setting. That is almost always
comfortable when I'm wearing a decent winter coat. If my wife/family are
with me I do need to increase that, but I still try to keep it below 70
degrees. Likewise, in the summer, I either just use the 'vent' setting or
set the temperature to something b/w 78-81 degrees (depends on how much
sunlight is shining directly into the vehicle as to what is comfortable
enough).
5)Weight in vehicle-- Although most small articles won't make a
major impact you don't wanna be carrying a bunch of unnecessary stuff in
the car all the time. If you always have the car full with the spouse and
3 kids and all their necessities of life in the hatch it will impact your
mileage.
6)Tire and road rolling resistance--The tire rolling resistance of
the OEM Goodyear Integrity tires is very low. If under inflated, however,
that resistance increases creating a significant impact on gas mileage.
Many of us have decided to use higher tire pressures than Toyota
recommends to further improve fuel economy by a small amount and to
improve tire wear, handling, and road hazard safety. Some argue that this
causes an uncomfortably rough ride and might be a safety issue. Whatever
pressure you choose to use always be sure to keep the 2psi higher pressure
in the front tires over the rear and to stay within the tire manufacture's
maximum rating (44psi for the Integrity).
Road rolling resistance is out of your control...but is a factor
none-the-less. If you have rough roads, wet roads, gravel, etc. that
increases rolling resistance and reduces mileage. Ideally dry smooth roads
are best.
7)Other weather related issues--Wind speed and direction have an
impact, as does rain (see above), humidity, elevation and barometric
pressure. I won't linger on those issues, but they do factor in.
8)Speed, braking, acceleration, and anticipatory driving--The MFD's
Energy and Consumption screens present a degree of feedback unprecedented
in any other stock vehicle. It acutely increases our awareness of what is
happening from moment to moment, every 5 minutes, and over time.
This has affected the way I drive dramatically and many other Prius
drivers notice the same. I accelerate fairly briskly getting to speed. I
don't floor it, but maybe depress the pedal 1/3rd way or so. Once I get
over the 25-30mph mark I begin trying to 'Dead-band'--on the energy screen
all power is from Engine (ICE) to wheels with no arrows to or from the
battery. This is the most efficient power phase as there are no losses
from converting the energy to potential energy in the battery back to
kinetic energy for the wheels.
I also try to anticipate stops well ahead of time. I've learned the timing
of the lights on my usual commute and can often see them far ahead and
know if I can make it by maintaining my current speed or if it will change
before I get there. I watch the other traffic as well trying to anticipate
what they might do I think that kind of anticipation makes for a safer
driver in addition to making it possible to drive more fuel efficiently.
When I know there is a stop say 1/2 mile ahead but there is traffic behind
me I'll go into a "glide" mode. I take my foot completely off the
accelerator for a second (you'll see just green arrows on the energy
screen) then I very lightly depress it again to a 'no arrows' condition on
the energy screen. That eliminates the 'drag' of the regenerative braking
that occurs with the foot completely off the pedal. If I need to maintain
a little more speed I'll press just enough to get the yellow arrows of
electric only power. Once I see that I must stop then I'll firmly, but not
hard, brake to try to use just regenerative braking (no friction brakes).
When possible I try to avoid coming to a complete stop with this
anticipatory method. If I can coast up to a red light and still be
traveling 10mph that is 10mph plus no static friction from a stop that I
have to use energy to get back.
Also, I almost never exceed the speed limit any more. The faster you go
the lower your fuel economy will be. Esp. at speeds over 60mph the impact
of air resistance, even in the low Cd Prius, is huge. Maintaining a steady
speed that doesn't impede traffic allows the Prius ECUs to find the most
efficient combination of ICE and Electric motor use.
As always, however, PLEASE make safety your first priority. When you
can safely take advantage of this type of anticipatory driving it is just
a bonus.
9)Fuel Type--Summer gas has a higher BTU rating than winter gas due
to pollution control issues in most parts of the US. Thus, you get more
energy per unit of gasoline with Summer gas. Thus, in addition to the cold
weather hit alone, now you get less 'bang-for-the-buck' from your
gasoline. Make it another 5% and you're down into the mid-40s mpg area
down from the 55mpg you may have gotten in the summer!! And it's
completely out of your control.
Also, many parts of the country are beginning to use E10 gas (10%
ethanol). This has an even lower BTU rating than winter gas (though has
environmental benefits in much lower sulfur content) and thus lower gas
mileage will be seen. Many people aren't even aware that they're using E10
fuel...the pump should have a small sticker notifying you of the fact.
10)Synthetic Oil--Some have stated they noticed a measurable
improvement in MPG just by switching to synthetic motor oil. I'm not sure
that this is true, but the synthetics do tend to have a slightly lower
viscosity than mineral oils so it's possible it has some effect. That
effect may be particularly noticeable in COLD weather where synthetic is
clearly a superior performer.
I, personally, chose to use synthetic after extensive research based
primarily on the slightly lower environmental impact of synthetic oil.
Also factored in was the potential for slightly higher gas mileage and
longer engine life. That has to be weighed against the significantly
higher cost for synthetic oil which certainly outweighs any fuel
efficiency improvements alone.
11)Short Trips--You'll almost immediately notice that your first 5
minute bar on the consumption screen will be in the range of 25-35mpg most
of the time. Extra fuel is used to warm up the engine and catalytic
converter in the first 5-10 minutes of driving. Thus, short trips are a
mileage killer. If you must take short trips try to combine them so that
there is minimal time for the ICE and CAT to cool down, thus maintaining
the warmth longer. The 'coolant' stored in the 'thermos' will also be
hotter to aid that process. Longer drives (over 15-20 min.) at moderate
speeds (40-45mph) will show some of your best gas mileage...often in the
60mpg range.
12)Terrain--Those of you lucky enough to live in flat areas like
New Orleans or Florida have a distinct advantage as well. The ECU is able
t maintain a steady speed over the entire distance traveled...The Cruise
control is truly your friend. Those of us who live in hilly or mountainous
terrain take a pretty good hit on the steeper hills. Although one can
sometimes recoup a little of the inefficiency on the down-hill portion,
it's never as much as was expended getting up the hill in the first place.
13)Alignment--For persistent issues with mileage not meeting your
expectations after factoring in ALL the items above, consider having your
alignment checked and fixed. This is pretty critical to long term mileage
and not always something that can be appreciated by 'feel' alone.
Summary
Multiple factors conspire to impact MPG and the relative impact seems
greater in a higher mileage vehicle like the Prius though, percentage
wise, it is probably not any worse than a conventional vehicle.
-Minimize AC use
-Maximize Anticipatory driving
-Consider higher than Toyota recommended tire pressures (many use 42f/40r,
40/38, or 38/36)
And finally: Be patient...it'll get better, there's nothing wrong with
your car. We've just all been set up with some relatively unrealistic
expectations and it's natural to be a little disappointed when those
expectations aren't immediately met.
LINKS AND REFERENCES
Another Great Resouce is
FuelEconomy.gov
A thread at the Yahoo site:
http://autos.groups.yahoo.com/group/Prius-2G/message/2742
See also the
Official Toyota PDF on Fuel Economy
Direct Link to PDF on PriusChat
Also this is a very thorough article from
Chevron on Fuel Economy that is well
worth downloading and reading.
Evan E. Fusco, MD |
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