Toyota Prius: Gas Mileage: Whom Do You Trust?

"Davoud" <net> wrote in message
news:280720060043120709%net... 

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.
mark_

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: 

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.

Paul

In article <googlegroups.com>,
"PriusGeek" <com> wrote:

 

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.

In article <280720060043120709%net>, Davoud <net>
wrote:
 

It is certainly not consistent. Thus one cannot assume that the amount
of fuel added at fillup number N is the same as the amount burned since
the previous fillup. Even if we assume that every gas pump is
completely accurate, this variability in the Prius gas tank capacity
makes a hash of trying to calculate MPG manually, except as an average
over many tanks.

And please don't top off. Topping off can saturate the fuel vapor
recovery canister and greatly increase emissions of unburned fuel from
the tank. Vapor recovery canisters have been around long enough that by
now topping off has become an old fart habit.

"richard schumacher" <net> wrote in message
news:isp.giganews.com... 

Thanks for telling us that Richard. This old fart will quit topping-off.
:-)

Something else to complicate your calculations. . .

You can't always trust the numbers you read off the gas pump. They can
be -- and sometimes are -- rigged to show more gasoline pumped than you
actually got. I've even heard in some cases they are even programmed
to be accurate up to the first five gallons, so people filling a
five-gallon can won't notice, then they start skewing the numbers.

Maybe it's just an urban myth, I've never set out to prove this
first-hand. But you know, gasoline scams have a long history (from
watered-down gas, to selling regular from the premium pump), so
jiggering the pump readings isn't far-fetched.

--
Tony Belding, Hamilton Texas

"PriusGeek" wrote ... 

I have recorded the pump MPG and the readout MPG for the last 35 fillups of my
2005. Averaging these values shows the displayed MPG to be about 1.25 mpg
higher than the MPG from the pump calculations. This difference has recently
come down a little bit, having run about 1.5 during the colder months.

My fillup end point has not been very consistent. The true calculated MPG has
differed from the Prius display value by as much as 6 mpg, but it is generally
much closer. The averaging should minimize this variability.

I have moderated my driving and the last 4 tanks have displayed over 55 mpg on
the screen. I doubt that the next fillup will achieve that goal.

-Al