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Gas Mileage: Whom Do You Trust?

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Posted by Davoud on July 28, 2006, 4:43 am

2006/package #8. Does one trust the digital display or the fill-up
quantity? I just returned from my first longish trip in the Prius (2450
mi.) It is my practice to top off the tank when refueling on such a
trip because I know that -- even on the Prius -- I will be burning it
off soon enough.

I suppose the question is whether the fuel bladder is consistent from
fill-up to fill-up -- will it fill to the same level each time if
topped off?

If the digital readout is to be believed I achieved 45.8 mpg highway;
the fill-up quantity indicated 51.3 highway.


usenet *at* davidillig dawt com

Posted by =?iso-8859-1?Q?mark=5Fdigital= on July 28, 2006, 7:05 am

A nearly six mile per gallon discrepancy over a span of 2450 miles is pretty
huge. Was the car listing when you filled it the last time? LOL
Trust your computer's mpg assessment. It's not affected by the compounding
of errors you get from each filling.

Posted by PriusGeek on July 28, 2006, 12:19 pm

Hmmmm.  I have measured the digital readout vs. calculated MPG at each
fillup for 2 Prii for over 100Kmiles (most on a "Classis 02") and have
found that the digital display typically runs about 5% higher on
average than actual calculated MPG (ie gallons put into tank divided by
miles driven on that tank of gas).  This seems to be true as well on my
06 that has about 8K miles on it so far.

With that said, the bladder is affected by temperature, altitude, the
rate of fuel coming from the pump and who knows what else.  On top of
that, while the pair of Prii I own are by far and away the absolute
best vehicles I have ever owned, I have to say the entire fuel gage /
MPG calculation system is inexplicably weak.  You would think that a
car that is pretty much completely computer controlled would be able to
measure and display fuel consumption exactly, but apparently Toyota
still uses the time honored tradition of guessing at fuel use by
monitoring engine vacuum pressure or something similar, which is not
very precise.  Oh, well.

When in doubt, use your calculated numbers.  Oh, and enjoy the Prius.
Did I mention its the best car I've ever owned?  ;-)

Davoud wrote:

Posted by Paul Russell on July 28, 2006, 1:18 pm

PriusGeek wrote:

I have a theory, with absolutely no evidence to back it up (but hey,
that's never stopped me before ;-)). My theory is that the Prius
calculates average MPG with respect to time rather than distance, which
tends to give a higher than expected value.

For exmaple, suppose you drive for 10 miles at 30 mph (20 minutes) with
an actual consumption rate of 60 mpg, followed by 10 miles at 60 mph (10
minutes) with an actual rate of 50 mpg. The "normal" way to calculate
the average consumption would be to to average over distance, which
would give 55 mpg. If my theory is correct then the Prius would
calculate an average figure of (20 * 60 + 10 * 50) / 30 = 56.7 mpg.


Posted by richard schumacher on July 28, 2006, 2:54 pm

The Prius counts fuel injector openings.  The fuel injectors must be
well-calibrated, otherwise combustion would not be well controlled and
the emissions would be much worse.  

An alternate explanation for this discrepancy is that the gas pump is
overstating the amount of fuel pumped.  Considering that this means
extra profit for the gas station, I decide to trust the Prius MFD.

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