MPG gauge.

Bailey B wrote:
> Some new cars have digital mpg sensors and gauges. For those of you
> that have them, how accurate do you think they are?
>
> When on interstate how much difference is their in mpg at 55 mph, 65
> mph, and 65 mph?
>
> A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg
> on stop and go gravel back roads.
>

Most of todays cars have computers that do the calculations,
using fuel flow to calculate the percentages in finite
detail. They're pretty accurate...
For instance, I own a 300Hp V6, sedan ( around 4000 #). On
Interstate highways, at 55, I get 27.5 mpg, 60 gives around
26, 65 around 25, etc..
A couple years ago, I rented a big caddy at Salt Lake City
to drive to Reno, Tahoe, etc. This was the first year they
mentioned $4.00/gal gas.. Being a bit cautious, I started
out westbound on US 80 toward Bonneville and would only
drive around 60 mph (or less), but I was sightseeing too.
I thought the fuel calculations were way off, but more than
once, the readout said 32mpg and it proved true when I
filled up. On the return, I kept the pedal down, to make my
plane out of SLC and it still got over 28. Of course this
is on mostly, very flat terrain. Much of your MPG will
depend on your driving habits..

On May 5, 1:38 am, [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?
>
> 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.

"John S." wrote:
>
> On May 5, 1:38 am, [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?
> >
> > 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.

--
Paul Hovnanian paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.

On May 7, 1:30 pm, "Paul Hovnanian P.E." <[email protected]> wrote:
> "John S." wrote:
>
> > On May 5, 1:38 am, [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?
>
> > > 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.
>
> --
> Paul Hovnanian  p...@hovnanian.com
> -----------------------------------------------------------------------
> Procrastinators: The leaders for tomorrow.- Hide quoted text -
>
> - Show quoted text -

It sure does. And it's a great tool for learning tricks to maximize
mpg too.

"John S." <[email protected]> wrote in message
news:47b98ee9-9f79-45ed-a401-929b0c925579@27g2000hsf.googlegroups.com...
> > 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.

It sure does. And it's a great tool for learning tricks to maximize
mpg too.

====

When I borrowed a friend's Golf TDI, which has an MPG meter, it was
interesting to see what did and didn't affect the reading.

- at a constant speed, if I relaxed the accelerator even very slightly, the
MPG figure shot up to amazingly good figures (eg 80-90); if I pressed the
accelerator even slightly beyond the amount needed to keep a constant speed,
it went down to about 40; at a steady 60 mph I was getting about 55 mpg (all
thse figures are per *UK* gallon: multiply by 4/5 to get US mpg)

- the consumption at a given speed seemed to be about the same irrespective
of whether the engine was hot (running for a long time) or cold (newly
started).

- changing up or down made surprisingly little difference: 50 in 4th or 4th
gear gave similar results with 5th only being slightly more efficient than
4th; however changing down in to 3rd (with the engine going at about 4000
rpm) was much less efficient, so there seems to be a cut-off engine speed
below which consumption is fairly constant but above which consumption
increases sharply

(probably both of the last two points are features of a diesel which might
not be true for a petrol engine)

- hard acceleration gave frighteningly low figures: a fairly brisk but not
wheel-spinning 0-60 through the gears, taking about 15 seconds, averaged
about 15 mpg :-(

- constant 30 was much much more efficient than slowing down to rest for a
junction and then accelerating away again; however as I came right off the
power when I was slowing down, I did get something like 300 mpg ;-)

- accelerating out of a roundabout in 3rd with a wider throttle setting was
more efficient than in 2nd with the engine going faster, for similar
acceleration rates - again, probably a diesel thing

John S. wrote:

>
> It sure does. And it's a great tool for learning tricks to maximize
> mpg too.

A manifold vacuum gauge is also a pretty handy tool for "training" you
to use less fuel. Lower manifold pressure generally equates to higher
mileage, unless carried to the point of idling away from traffic
lights... ;-)

Paul Hovnanian P.E. wrote:
> "John S." wrote:
>> On May 5, 1:38 am, [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.

--
NOTE: to reply, remove all punctuation from email name field

Ned Forrester [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