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Question about MPG - Page 9

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Posted by Bill on August 15, 2006, 8:52 am


The prius recovers energy whether coasting to a stop or braking to a stop.

 >This is evidenced by the very low capacity  of the Prius battery pack,
only able to power the >car for a few seconds at  full output, which in turn
explains why Toyota provided no facility to

What is evidenced by the battery packs capacity?  The battery pack stores
recovered energy.  It has the capacity to do precisely what Toyota intended.

Of course, so does lugging around a diesel engine.

Steve, this discussion would make more sense if you gave us the make and
model of the particular diesel you are comparing to the Prius.  Having that,
we can compare the specifications of  that vehicle with the specifications
of the Prius.   Let's face it, you have made a lot of claims in this
discussion but you are the only one who possesses  both sets of

Posted by Steve Pardoe on August 15, 2006, 5:23 pm

I understand 'coasting to a stop' as being what happens when you release the
accelerator (gas) pedal - on a level road, the car slows gently owing to
friction from the air, rotating surfaces and tyre hysteresis, and ultimately
engine braking, depending on the gear selected (I know the Prius has CVT so
that's not applicable in the same way as a normal gearbox).  'Braking' is
deliberate slowing of a much greater magnitude.  I'd be surprised if the
Prius regenerated when 'coasting', since this would inevitably slow the car
(like deliberate engine braking does) much more than just taking the foot
off the pedal.

Quite, which is just enough to give a short acceleration boost or run some
utilities with the IC engine off, but not enough to operate as a true
electric car for any distance.  This isn't a criticism, just a
clarification.  Believe it or not, there are people out there who think the
battery somehow contributes a net positive energy to the car.

Not to the same extent, the weight premium for a modern diesel (even
including its heavier starter, turbo, etc) over a comparable-output gasoline
engine is much less than the Prius battery pack etc.


I'm not actually making a comparison with a specific diesel, just trying to
find out (see my first article in this thread) whether the mpg figures
claimed by Toyota and some (naturally) pro-Prius subscribers to this forum
are truly based on real measurements of road mileage and of fuel consumed
(measured by what you put in the tank), or whether they are just what the
'computer' (OK, not a trip computer) on the dash says.  That's all.

Clearly, there will be people (especially those who have bought Prius cars)
who are very anxious to claim an optimistic figure.  All I'm pointing out is
that if you want really good fuel efficiency, we've been doing it in Europe
with diesel cars such as the '3 litre' VW Lupo, for years.  That's three
litres per 100km which equates to about 94 mpg (imperial) or, say, 75 mpg
(US gallon), and this is readily achievable even on long-distance driving.
VW are bringing out even more economical models.  In contrast, quoting from
Wikipedia,  "By the European method, the combined fuel economy of the Prius
is 4.3 L/100 km or 55mpg (US)".


Again it's not a matter of specifications but of principle.  I'm just
curious to know whether the claims of fuel economy are based on real
numbers, or the say-so of a dial on the dash.

For the record, my own car is a Skoda Octavia with a 2-litre diesel.
Keeping a log of every litre of fuel I've ever put in it, over 13,438 miles
it's used 1,340 litres and so averaged 42.2 mpg (imperial).  This is a good
bit worse than a Prius, but it's a much bigger and more capable car, and at
least I know that the figure is accurate (as accurate as the fuel pumps at
the forecourt and the odometer, which I have checked against the motorway
mileposts on a long run).  By comparison, the trip computer reads about 7%
optimistic on average.


Posted by Bill on August 15, 2006, 5:58 pm

You should drive a Prius.  Toyota did a great job.  The "feel" of the
charging that takes place while coasting to a stop is identical to the feel
of engine braking in a conventional car.  A "B" (engine braking) shift
position is provided if actual engine braking is needed for long hills.
Yes, I was surprised too.

No, it can drive as a true electric car for a significant distance.  We
Prius owners call this the "stealth mode".   Frankly, you need to rent one
for a few days.  From what you write here it is clear you are describing how
you *think* the Prius works, not how it does work.

The tiny, 3-door Lupo hardly compares to the mid-sized, 5-door Prius.

I tried.  They aren't comparable, one being a mini, the other being a mid.
Couldn't find the Lupo emission figures.

Let's face it, you have made a lot of

Yes, it is a matter of specifications.  Not every family of five will fit on
a mo-ped.
Rent one.   You are trying to compare apples to oranges but you've never
eaten an orange.

Posted by Steve Pardoe on August 15, 2006, 7:02 pm


I can't fault your logic - I am speaking from what I've read, not first hand
experience.  However, despite our interesting debate (for which I thank you)
I still haven't seen the real-world figures requested by the OP.


Posted by Bill on August 15, 2006, 7:44 pm

Steve, "real world figures" are anecdotal and largely depend on individual
driving skills and habits.  Your real world differs from mine.  The same
Prius will produce much better mileage for the husband than for the wife or
visa versa.  Where I live many of the highways have a 55 mph speed limit.
This is very close to the speed at which our Environmental Protection Agency
tests cars to determine the figures they publish, the ones that are by law
posted on the window of every new car.  If you drive 55 you will, on
average, achieve the published figures.

As a consequence, the only fair comparison is of the EPA figures for
vehicles of the same size.  Deviations from the published figures will
affect both vehicles.  In fact, a number of people have "tested" the Prius
and concluded it does not achieve the EPA figures in their "real world"
scenario.  They tested the Prius because they were astounded by the
published figures and you will note they don't compare their "real world"
results with the "real world" results of some other car that underwent an
identical "real world" test.  The same driver's "real world" would have a
proportional effect on whatever car they tested.  Also, a number of people
have made a hobby out of exceeding the EPA figures.  It is important to
remember that a 10% reduction from 50 mpg is 5 mpg while a 10% reduction
from 20 mpg is 2 mpg.  10% is 10%, but some people can't comprehend that.

I've concluded that publishers who whine about this "real world" of theirs
are people who live in a world that doesn't care about either pollution or
conservation.  People who do will meet or beat the published figures.  After
all, this is what "Environmental Protection" is all about.

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