Paul Hovnanian P.E. wrote:
> "John S." wrote:
>> On May 5, 1:38 am, Baile...@[EMAIL PROTECTED]
(Bailey B) wrote:
>>> Some new cars have digital mpg sensors and gauges.
>> Actually most new cars either have them as stanrd or as an option now.
>>
>>> For those of you
>>> that have them, how accurate do you think they are?
In our Chevy Astro, we took an 8000mi trip. I used the records of fuel
purchased during the trip and compared that with the total accumulated
fuel that the computer claimed was burned. The comparison was (to me)
surprisingly close. I don't recall the exact number, my recollection
was that the numbers agreed within a percent or two.
As far as I know, the computer makes all the fuel calculations (mpg,
total fuel, etc.) based on injector pulse width. Even though the
computer is mainly regulating fuel flow based on feedback from the
oxygen sensor (and therefore extreme precision is not required), it
seems to have a pretty accurate direct measure of fuel from pulse width
and known fuel pressure. Of course, the accuracy of MPG is also
affected by measurement of speed/distance; the primary uncertainty for
that measurement is the tire diameter. Any changes from standard tires
or axle ratio would require some sort of reprogramming to maintain
accuracy.
>>> When on interstate how much difference is their in mpg at 55 mph, 65
>>> mph, and 65 mph?
>> Within those ranges MPG will go down down as MPH goes up. That is
>> pretty much a given.
>>
>>> A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg
>>> on stop and go gravel back roads.
>> That makes sense to me.
>>
>> MPG computers are very acccurate in my experience if you measure
>> average mpg over a full tank. The spot number also appears to be
>> quite acccurate.
>
> The spot number also gives you a pretty good idea of how much gas
> certain bad driving habits cost you.
--
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Ned Forrester n_f_orrester@[EMAIL PROTECTED]
508-289-2226
Applied Ocean Physics and Engineering Dept.
Oceanographic Systems Lab http://adcp.whoi.edu/
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
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