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Posted by Paul Russell on July 28, 2006, 9:18 am
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PriusGeek wrote:
> 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.
>
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
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