Hybrid Car – More Fun with Less Gas

Gas Mileage: Whom Do You Trust? - Page 6

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Posted by PriusGeek on August 1, 2006, 12:19 pm

Well, my 92K mile experience would differ.  While there  are
variabilities in individual fillups, over the span of 3 years where I
logged each of 203 fillups and the number of miles between them and
found that the MPG display in the Prius was 5.21% higher than the
calculated MPG. It was pretty consistent; a few times higher and a few
times lower.  I have to believe that over 203 tankfulls at all sorts of
gas stations, that the errors due to top offs, temperature changes,
pump accuracy etc. should be pretty much netted out.  At the end of the
day, the true cost of fuel for the car is what I actually pay for it
and pump into the car, and not what the meter in  the car might tell
me. Dividing that by the number of miles I drove as shown on the
odometer will yield the true MPG.

Curtis CCR wrote:

Posted by Miwaku on August 1, 2006, 1:59 pm

PriusGeek wrote:

Yes, the true cost of the fuel depends on what you pay at the pump -
which has nothing whatsoever to do with the original question.

Everyone has their own opinion. After posting my pump tracking
spreadsheet in the PriusChat forums, John1701 (a well-known Prius
blogger) challenged that the only true measurement of MPG is the
hand-calculated method, not the MFD, and suggested I view his 114k+
miles of records. I did so, plugged them into my own spreadsheet, and
found out that his lifetime MPG were less than 2mpg off from hand to

While some Prius geeks (not talking about you, PriusGeek!) track fuel,
mileage, cost, and MFD readings, I also happen to track fuel pips on
the "guess" gauge. For the most part, the first pip disappeared after
anywhere from 110-160 miles, and all the rest dropped after 20-60
miles. The one time I got GREAT mileage for me (56), the first pip
dropped at 50, and the next tank showed crappy mileage (36).
Translation: I didn't fill up all the way prior to the great mileage.
Amazingly, my MFD said both tanks were about the same as far as MPG.
You guys can debate this all day, talking about teh 9/10 of a cent
issue, the fuel injector issue, the gas pump issue, and the "I know I
filled it all the way up, even though I'm at a different gas station
using a different model of pump. The only constant in this is the Prius
and its MFD, so that's what I'm going with. Somehow, I think it's
slightly more accurate than a commercial gas pump or any driver on the
road, but then, I tend to trust computers. They're pretty good at that
cipherin' stuff...

Posted by Bill on August 2, 2006, 4:00 am

Right.  Your readings and calculations are valid for your car.    Isn't
there a site where a wide range of such readings and calculations have been
posted?  Damned analog devices.  Hard to get consistency in mass production.

Posted by A Sherman on August 2, 2006, 12:11 pm

No, not an analog issue (except for the fill-ups).

How come PriusGeek reports a 5.2% difference and I get about 3% for the same
calculation?  I have a 2005.  Is the "classic Prius" different?

Does anyone know exactly how the MPG display is calculated?  Is it the
calculation of the total fuel through the injectors divided by the distance
covered, or some kind of a weighted average?


Posted by PriusGeek on August 2, 2006, 5:17 pm

I'm sure that individual driving styles as well as differences between
vehicles (tire wear, brakes, etc.) accounts for some of the
differences.  I keep the same data on my '06, and over 16 tanks, my
variance is 2.7% (display more optimistic than the calculated value)
But that's not the whole story, as I have several tankfulls where the
variance was as much as +/- 10%, so it is important to look at the data
over a significant number of fillups.

On the other hand, my 02 Classic was the best car I ever owned until I
bought my 06! For more perspective  I get infinite pleasure filling up
with 10 gallons after 500 miles of travel while the Hummer next to me
is maxing out his credit card after a couple of hundred miles.

A Sherman wrote:

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