On 09/17/2013 09:07 AM, Oleg Goldshmidt wrote:
guy keren <guy.choo.ke...@gmail.com> writes:
"watching the numbers occasionally" is not a proper experiment. you
need to reset the computer before you start the "drive under test",
and check the value after - and the length should be enough to even
out the fluctuations.
I don't know what your car shows you. Mine directly shows the fuel
consumption at the moment (that jumps around) and a running average over
some period of time that is updated every few seconds. I don't remember
what the averaging period is exactly, but if you drive at a steady speed
for a while (I have cruise control, too) you will get a pretty stable
number on the screen.
watching the "current consumption" numbers can be quite missleading,
since' during a lengthy period of drive, the number is usually not
stable, and the assumed summing up of the numbers isn't necessarily the
real summing up of the numbers.
These are two different screens on the dashboard that I can switch
between with a button on the steering wheel. The computer screen is
right next to the speedometer, so I can watch the speed (even without
cruise control) and fuel consumption simultaneously. In principle, I
think there is another screen that reports your running average speed,
but I don't think I used it for this purpose.
I don't know what you mean by "resetting" the computer. I assume you
reset the "trip distance" counter. I don't even need it to watch the
fuel consumption numbers.
one of the fuel consumption parameters the Prius gives (and it also
existed in the renault megan i had on year 2000) is the "average fuel
consumption since the last reset" - and you can manually reset this
counter whenever you want - so it allows you to reset the counter,
perform a drive of any distance you wish (1km or a million km - doesn't
manner) - and get the actually (computed, not guesstimated) fuel
consumption you had across the entire drive. to me - this is the *only*
number that counts, since the other numbers are not steady enough across
a long drive.
It sounds like you took a trip with a full tank, guesstimated your
average velocity, and topped the tank again to see how much fuel you
spent. If I misread, sorry. If this is roughly what you did, then I am
sorry to say I am not particularly impressed with the methodology (I
realize this is the only thing you may be able to do - no offence meant
at all - it is better than nothing). It cannot possibly be close in
precision or reliability to direct observation of km/l or l/km.
as i said - i let the car's computer (together with the resetting i
mentioned). since i also have cruise control in the car, i can also
assure a fixed speed across a given distance.
and by the way, this fixed speed does NOT generate fixed fuel
consumption across a long drive. very tiny changes in the road's slope
(even on what superficially appear to be a flat road) bring it up and
down quite dramatically.
in fact, driving without cruise control and adjusting the speed to the
changing road conditions allows you better fuel consumption then using
cruise control. since the Prius's speed meter is digital rather then
analog, you can see exactly how a change of speed of even 1km/h
sometimes has a dramatic effect on the fuel consumption. even more -
sometimes keeping the same speed but slightly changing the pressure
level on the accelerator - can change the fuel consumption considerably.
I hope the above gives you a good idea how I know. This is the least
"theoretical" approach mentioned so far. All my "occasional
observations" disclaimers mean that I didn't obsessively do it over
dozens of trips, write down the numbers, run F-tests or whatever...
my observations showed me that what i guessed to be the fuel consumption
based on watching the "current consumption number", and what i actually
used across a 5 minutes period, or across a distance, can be completely
different number, and thus the former is not a useable measure if you
want to know how much fuel you've eventually used across a distance.
--guy
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