Gas Tank Level Theory

I have been watching my 91 Civic's mileage particularly
closely since about October. This includes, for the
overenthused, watching the fuel tank gage. A few times I
have thought to myself, "Darn, it's reading just about
half-full, and I usually have at least X miles by this
point. The trip odometer is at more like X-50 miles right
now." So I would predict that the next fillup will yield
stats indicating really bad mileage. But on the contrary,
apart from a few weeks where I had the timing messed up, it
looks good, for winter.

As people have indicated here recently, gas pumped in the
summer from a nice cool underground tank (typically) expands
once in the car's tank and while warming to ambient. This
makes sense. In this vein, could it be that, while driving
in the summer, the fuel tank gage reads particularly
disproportionately to the lbs. of fuel consumed? That is,
the actual level in the fuel tank goes down literally more
slowly from full tank to half, because the gasoline in the
tank is simultaneously expanding (due to temperature
increases). By the time the driver reaches a half tank or
so, the gasoline isn't expanding as quickly, because its
temperature is pretty constant.

I recall times in the summer where my Civic has achieved
nearly 300 miles by the time the gage reads half full. Then
it drops very quickly. In winter, I can't get anywhere near
as many miles on the trip odometer by the time the tank is
half full. Still great mileage; just totally out of whack
with the fuel gage.

Anyone else notice this? Comments on this theory?

Elle wrote:
> Anyone else notice this? Comments on this theory?

isnt there a potentiometer in the tank operating the gauge that could be
kinda wonky?

SoCalMike wrote:
> Elle wrote:
> > Anyone else notice this? Comments on this theory?
>
> isnt there a potentiometer in the tank operating the gauge that could be
> kinda wonky?

As Mike indicates, the fuel gage inside the tank can be bad but it
could just be due to temperature in the tank.

Since tank level indicators are often just wirewound potentiometers,
they do fluctuate with temperature. A potentiometer is a variable
resistance. Depending on the applications, they can just wire wound
around a bobbin. This wire's resistance changes with temperature,
depends on the wire used, etc.
If the circuit that drives it fluctuates at a different rate, in a
different direction or not at all, you may read different results at
different temperatures.

To eliminate the fuel gauge as an indicator of how much gas you are
using, maybe just start a log, writing down how much you fill up,
current mileage and outside temperature.
Your actual MPG = (mileage reading at fill up - last mileage reading
at fill up) / gallons filled with.
See if this number fluctuates significantly with temperature.

Remco

Elle wrote:
>
> I have been watching my 91 Civic's mileage particularly
> closely since about October. This includes, for the
> overenthused, watching the fuel tank gage. A few times I
> have thought to myself, "Darn, it's reading just about
> half-full, and I usually have at least X miles by this
> point. The trip odometer is at more like X-50 miles right
> now." So I would predict that the next fillup will yield
> stats indicating really bad mileage. But on the contrary,
> apart from a few weeks where I had the timing messed up, it
> looks good, for winter.
>
> As people have indicated here recently, gas pumped in the
> summer from a nice cool underground tank (typically) expands
> once in the car's tank and while warming to ambient. This
> makes sense. In this vein, could it be that, while driving
> in the summer, the fuel tank gage reads particularly
> disproportionately to the lbs. of fuel consumed? That is,
> the actual level in the fuel tank goes down literally more
> slowly from full tank to half, because the gasoline in the
> tank is simultaneously expanding (due to temperature
> increases). By the time the driver reaches a half tank or
> so, the gasoline isn't expanding as quickly, because its
> temperature is pretty constant.
>
> I recall times in the summer where my Civic has achieved
> nearly 300 miles by the time the gage reads half full. Then
> it drops very quickly. In winter, I can't get anywhere near
> as many miles on the trip odometer by the time the tank is
> half full. Still great mileage; just totally out of whack
> with the fuel gage.
>
> Anyone else notice this? Comments on this theory?


Yes, I would agree that winter vs. summer driving would see differences.

However, If you are now noticing the change becoming more apparent, the
sender for the gas gauge is probably starting to go. I base this
experience on my daily drivers that generally forty to fifty years old
and rather than trust the gauge, I simply watch the odometer and fill up
at ever "x" amount of miles.

There are ways to "renew" the sender but I believe that in most cases
(for our purposes), it is part of the fuel pump on Hondas. Is this
assumption correct? If so, better to let sleeping dogs lie than disturb
it and awaken other symptoms. Regarding "renewal," I would suggest
rooting around some of the antique groups for the procedure if that is
what you choose to do...

JT

"Remco" > wrote
> SoCalMike wrote:
> > Elle wrote:
> > > Anyone else notice this? Comments on this theory?
> >
> > isnt there a potentiometer in the tank operating the
gauge that could be
> > kinda wonky?
>
> As Mike indicates, the fuel gage inside the tank can be
bad but it
> could just be due to temperature in the tank.

I couldn't rule that out, sure.

> Since tank level indicators are often just wirewound
potentiometers,
> they do fluctuate with temperature. A potentiometer is a
variable
> resistance. Depending on the applications, they can just
wire wound
> around a bobbin. This wire's resistance changes with
temperature,
> depends on the wire used, etc.
> If the circuit that drives it fluctuates at a different
rate, in a
> different direction or not at all, you may read different
results at
> different temperatures.
>
> To eliminate the fuel gauge as an indicator of how much
gas you are
> using, maybe just start a log, writing down how much you
fill up,
> current mileage and outside temperature.
> Your actual MPG = (mileage reading at fill up - last
mileage reading
> at fill up) / gallons filled with.

This is what I've been doing for a couple years or so now.

I'm not worried about the fuel gage. Just seems to me that
it does not read in direct proportion to what's in the tank,
and it may be worse in the summer.

> See if this number fluctuates significantly with
temperature.

But winter driving conditions affect mileage IMO. So I get
pretty consistently above 40 mpg in the summer, and
sometimes 39 mpg or so in winter, or maybe right at 40 mpg.
It definitely declines in cooler weather, but IMO that's due
to all the usual reasons much discussed here.

"Grumpy AuContraire" > wrote
> Elle wrote:
> >
> > I have been watching my 91 Civic's mileage particularly
> > closely since about October. This includes, for the
> > overenthused, watching the fuel tank gage. A few times I
> > have thought to myself, "Darn, it's reading just about
> > half-full, and I usually have at least X miles by this
> > point. The trip odometer is at more like X-50 miles
right
> > now." So I would predict that the next fillup will yield
> > stats indicating really bad mileage. But on the
contrary,
> > apart from a few weeks where I had the timing messed up,
it
> > looks good, for winter.
> >
> > As people have indicated here recently, gas pumped in
the
> > summer from a nice cool underground tank (typically)
expands
> > once in the car's tank and while warming to ambient.
This
> > makes sense. In this vein, could it be that, while
driving
> > in the summer, the fuel tank gage reads particularly
> > disproportionately to the lbs. of fuel consumed? That
is,
> > the actual level in the fuel tank goes down literally
more
> > slowly from full tank to half, because the gasoline in
the
> > tank is simultaneously expanding (due to temperature
> > increases). By the time the driver reaches a half tank
or
> > so, the gasoline isn't expanding as quickly, because its
> > temperature is pretty constant.
> >
> > I recall times in the summer where my Civic has achieved
> > nearly 300 miles by the time the gage reads half full.
Then
> > it drops very quickly. In winter, I can't get anywhere
near
> > as many miles on the trip odometer by the time the tank
is
> > half full. Still great mileage; just totally out of
whack
> > with the fuel gage.
> >
> > Anyone else notice this? Comments on this theory?
>
>
> Yes, I would agree that winter vs. summer driving would
see differences.
>
> However, If you are now noticing the change becoming more
apparent, the
> sender for the gas gauge is probably starting to go. I
base this
> experience on my daily drivers that generally forty to
fifty years old
> and rather than trust the gauge, I simply watch the
odometer and fill up
> at ever "x" amount of miles.

I wouldn't say it's more apparent. I remember years ago
taking vacation trips in the car and getting this amazing
mileage for the first half tank, suggesting I was going to
get like 600 miles out of a full tank. Then the decline
would be rapid, and I'd get the usual 390-450 miles from a
full tank, say.

But thanks for the tip, should things ever go way out of
whack.

> There are ways to "renew" the sender but I believe that in
most cases
> (for our purposes), it is part of the fuel pump on Hondas.
Is this
> assumption correct? If so, better to let sleeping dogs
lie than disturb
> it and awaken other symptoms. Regarding "renewal," I would
suggest
> rooting around some of the antique groups for the
procedure if that is
> what you choose to do...

Oh, I'm not complaining. Just wondering, since some folks
have occasionally come here ISTM and asked about the
accuracy of the fuel tank gage.

Elle wrote:
> "Remco" > wrote
> > SoCalMike wrote:
> > > Elle wrote:
> > > > Anyone else notice this? Comments on this theory?
> > >
> > > isnt there a potentiometer in the tank operating the
> gauge that could be
> > > kinda wonky?
> >
> > As Mike indicates, the fuel gage inside the tank can be
> bad but it
> > could just be due to temperature in the tank.
>
> I couldn't rule that out, sure.
>
> > Since tank level indicators are often just wirewound
> potentiometers,
> > they do fluctuate with temperature. A potentiometer is a
> variable
> > resistance. Depending on the applications, they can just
> wire wound
> > around a bobbin. This wire's resistance changes with
> temperature,
> > depends on the wire used, etc.
> > If the circuit that drives it fluctuates at a different
> rate, in a
> > different direction or not at all, you may read different
> results at
> > different temperatures.
> >
> > To eliminate the fuel gauge as an indicator of how much
> gas you are
> > using, maybe just start a log, writing down how much you
> fill up,
> > current mileage and outside temperature.
> > Your actual MPG = (mileage reading at fill up - last
> mileage reading
> > at fill up) / gallons filled with.
>
> This is what I've been doing for a couple years or so now.
>
> I'm not worried about the fuel gage. Just seems to me that
> it does not read in direct proportion to what's in the tank,
> and it may be worse in the summer.
>

Chances are they are not linear and probably gets worse with
temperature.

One project I did was for TLIs (Tank Level Indicators) for use on subs
and surface fleet. The problem was that the tank readouts are
notoriously non-linear and very inaccurate because of environmental
fluctuations.
Tanks are round, usually. NAVY tanks have stuff like pipes running
thought them. So the fill curve looks like a mountain ridge - peaks and
valleys. Cars are perhaps less so, but certainly they are not square
boxes (an ideal linear tank) so their tank curve is not linear.

It was so bad that when they were filling a tank, they'd as a rule
spill oil, causing environmental problems and be fined. The NAVY is
interested in filling FAST and accuracy is a distant second - remember,
they can be in war like conditions.
Of course, they are interested what they have left in their tanks. The
standard NAVY carrier procedure to check tank level is to drop a plumb
bobbin with a string into a tank, to not trust the TLI (incredible, but
true).
The stuff I designed alleviated that problem under most circumstances.

So if the NAVY is having problems, one can only imagine how bad an
automotive system is.

>
> But winter driving conditions affect mileage IMO. So I get
> pretty consistently above 40 mpg in the summer, and
> sometimes 39 mpg or so in winter, or maybe right at 40 mpg.
> It definitely declines in cooler weather, but IMO that's due
> to all the usual reasons much discussed here.

Could be - Never looked into that. You could very well be right. In
cooler weather one does run the heater/lights and nothing comes free.
Of course, AC in summer costs more so one would expect summer driving
to be worse, when it comes to mileage...

It would be an interesting thing if we all kept track of our mileage
and posted it somewhere, along with current temperature. We'd get this
very large mileage database of mileage change of cars against
temperature. Obviously we would not be interested in the actual
mileage, but the change in mileage against temperature. (here's the
scientist talking - it may not be practical

We might find out that maybe the oil companies add something to their
gas in winter or summer we don't know about. Maybe humidity of air
affects the gas somehow? Just guessing...

Elle wrote:>
>> snipped<<
>Still great mileage; just totally out of whack
> with the fuel gage.
>
> Anyone else notice this? Comments on this theory?


The simple fact is, the fuel gauge is just that... a gauge, not a
calibrated instrument, as say an altimeter would be. Now if the fuel
level gauge was based on fuel trim or other engine management inputs,
that would be a different story.

"Remco" > wrote
> Elle wrote:
> > "Remco" > wrote
> > > SoCalMike wrote:
> > > > Elle wrote:
> > > > > Anyone else notice this? Comments on this theory?
> > > >
> > > > isnt there a potentiometer in the tank operating the
> > gauge that could be
> > > > kinda wonky?
> > >
> > > As Mike indicates, the fuel gage inside the tank can
be
> > bad but it
> > > could just be due to temperature in the tank.
> >
> > I couldn't rule that out, sure.
> >
> > > Since tank level indicators are often just wirewound
> > potentiometers,
> > > they do fluctuate with temperature. A potentiometer is
a
> > variable
> > > resistance. Depending on the applications, they can
just
> > wire wound
> > > around a bobbin. This wire's resistance changes with
> > temperature,
> > > depends on the wire used, etc.
> > > If the circuit that drives it fluctuates at a
different
> > rate, in a
> > > different direction or not at all, you may read
different
> > results at
> > > different temperatures.
> > >
> > > To eliminate the fuel gauge as an indicator of how
much
> > gas you are
> > > using, maybe just start a log, writing down how much
you
> > fill up,
> > > current mileage and outside temperature.
> > > Your actual MPG = (mileage reading at fill up - last
> > mileage reading
> > > at fill up) / gallons filled with.
> >
> > This is what I've been doing for a couple years or so
now.
> >
> > I'm not worried about the fuel gage. Just seems to me
that
> > it does not read in direct proportion to what's in the
tank,
> > and it may be worse in the summer.
> >
>
> Chances are they are not linear and probably gets worse
with
> temperature.
>
> One project I did was for TLIs (Tank Level Indicators) for
use on subs
> and surface fleet. The problem was that the tank readouts
are
> notoriously non-linear and very inaccurate because of
environmental
> fluctuations.
> Tanks are round, usually. NAVY tanks have stuff like pipes
running
> thought them. So the fill curve looks like a mountain
ridge - peaks and
> valleys. Cars are perhaps less so, but certainly they are
not square
> boxes (an ideal linear tank) so their tank curve is not
linear.

Obnoxious aside: I actually do have experience with
submarine tanks, but only the ones in the engine room, which
were quite a bit smaller and were not being fully emptied
and filled on a regular basis, or a direct reading level
gage sufficed, if they were.

> It was so bad that when they were filling a tank, they'd
as a rule
> spill oil, causing environmental problems and be fined.
The NAVY is
> interested in filling FAST and accuracy is a distant
second - remember,
> they can be in war like conditions.
> Of course, they are interested what they have left in
their tanks. The
> standard NAVY carrier procedure to check tank level is to
drop a plumb
> bobbin with a string into a tank, to not trust the TLI
(incredible, but
> true).
> The stuff I designed alleviated that problem under most
circumstances.
>
> So if the NAVY is having problems, one can only imagine
how bad an
> automotive system is.

Good anecdote.

> > But winter driving conditions affect mileage IMO. So I
get
> > pretty consistently above 40 mpg in the summer, and
> > sometimes 39 mpg or so in winter, or maybe right at 40
mpg.
> > It definitely declines in cooler weather, but IMO that's
due
> > to all the usual reasons much discussed here.
>
> Could be - Never looked into that. You could very well be
right. In
> cooler weather one does run the heater/lights and nothing
comes free.
> Of course, AC in summer costs more so one would expect
summer driving
> to be worse, when it comes to mileage...

Not to lecture, but there's a whole slew of other items that
afaic have been reasonably proposed in the past, like the
engine ECU runs the engine at idle longer or slightly more
rich; the oil viscosity is higher, the air is denser so wind
resistance is higher in winter, etc.

>
> It would be an interesting thing if we all kept track of
our mileage
> and posted it somewhere, along with current temperature.
We'd get this
> very large mileage database of mileage change of cars
against
> temperature. Obviously we would not be interested in the
actual
> mileage, but the change in mileage against temperature.
(here's the
> scientist talking - it may not be practical
>
> We might find out that maybe the oil companies add
something to their
> gas in winter or summer we don't know about. Maybe
humidity of air
> affects the gas somehow? Just guessing...

SoCalMike and Jim Beam have pointed out that, at least in
some parts of the country, the gasoline formulation for
winter is different, and of a lower calorific value, than in
the warmer months. Various fed and state EPA web sites back
this up, as well. So for some drivers, for sure this is a
factor partly explaining their lower gas mileage around
winter.

Elle wrote:
> "Remco" > wrote
> > Elle wrote:
> > > "Remco" > wrote
> > > > SoCalMike wrote:
> > > > > Elle wrote:
> > > > > > Anyone else notice this? Comments on this theory?
> > > > >
> > > > > isnt there a potentiometer in the tank operating the
> > > gauge that could be
> > > > > kinda wonky?
> > > >
> > > > As Mike indicates, the fuel gage inside the tank can
> be
> > > bad but it
> > > > could just be due to temperature in the tank.
> > >
> > > I couldn't rule that out, sure.
> > >
> > > > Since tank level indicators are often just wirewound
> > > potentiometers,
> > > > they do fluctuate with temperature. A potentiometer is
> a
> > > variable
> > > > resistance. Depending on the applications, they can
> just
> > > wire wound
> > > > around a bobbin. This wire's resistance changes with
> > > temperature,
> > > > depends on the wire used, etc.
> > > > If the circuit that drives it fluctuates at a
> different
> > > rate, in a
> > > > different direction or not at all, you may read
> different
> > > results at
> > > > different temperatures.
> > > >
> > > > To eliminate the fuel gauge as an indicator of how
> much
> > > gas you are
> > > > using, maybe just start a log, writing down how much
> you
> > > fill up,
> > > > current mileage and outside temperature.
> > > > Your actual MPG = (mileage reading at fill up - last
> > > mileage reading
> > > > at fill up) / gallons filled with.
> > >
> > > This is what I've been doing for a couple years or so
> now.
> > >
> > > I'm not worried about the fuel gage. Just seems to me
> that
> > > it does not read in direct proportion to what's in the
> tank,
> > > and it may be worse in the summer.
> > >
> >
> > Chances are they are not linear and probably gets worse
> with
> > temperature.
> >
> > One project I did was for TLIs (Tank Level Indicators) for
> use on subs
> > and surface fleet. The problem was that the tank readouts
> are
> > notoriously non-linear and very inaccurate because of
> environmental
> > fluctuations.
> > Tanks are round, usually. NAVY tanks have stuff like pipes
> running
> > thought them. So the fill curve looks like a mountain
> ridge - peaks and
> > valleys. Cars are perhaps less so, but certainly they are
> not square
> > boxes (an ideal linear tank) so their tank curve is not
> linear.
>
> Obnoxious aside: I actually do have experience with
> submarine tanks, but only the ones in the engine room, which
> were quite a bit smaller and were not being fully emptied
> and filled on a regular basis, or a direct reading level
> gage sufficed, if they were.
>
> > It was so bad that when they were filling a tank, they'd
> as a rule
> > spill oil, causing environmental problems and be fined.
> The NAVY is
> > interested in filling FAST and accuracy is a distant
> second - remember,
> > they can be in war like conditions.
> > Of course, they are interested what they have left in
> their tanks. The
> > standard NAVY carrier procedure to check tank level is to
> drop a plumb
> > bobbin with a string into a tank, to not trust the TLI
> (incredible, but
> > true).
> > The stuff I designed alleviated that problem under most
> circumstances.
> >
> > So if the NAVY is having problems, one can only imagine
> how bad an
> > automotive system is.
>
> Good anecdote.
>
> > > But winter driving conditions affect mileage IMO. So I
> get
> > > pretty consistently above 40 mpg in the summer, and
> > > sometimes 39 mpg or so in winter, or maybe right at 40
> mpg.
> > > It definitely declines in cooler weather, but IMO that's
> due
> > > to all the usual reasons much discussed here.
> >
> > Could be - Never looked into that. You could very well be
> right. In
> > cooler weather one does run the heater/lights and nothing
> comes free.
> > Of course, AC in summer costs more so one would expect
> summer driving
> > to be worse, when it comes to mileage...
>
> Not to lecture, but there's a whole slew of other items that
> afaic have been reasonably proposed in the past, like the
> engine ECU runs the engine at idle longer or slightly more
> rich; the oil viscosity is higher, the air is denser so wind
> resistance is higher in winter, etc.
>
> >
> > It would be an interesting thing if we all kept track of
> our mileage
> > and posted it somewhere, along with current temperature.
> We'd get this
> > very large mileage database of mileage change of cars
> against
> > temperature. Obviously we would not be interested in the
> actual
> > mileage, but the change in mileage against temperature.
> (here's the
> > scientist talking - it may not be practical
> >
> > We might find out that maybe the oil companies add
> something to their
> > gas in winter or summer we don't know about. Maybe
> humidity of air
> > affects the gas somehow? Just guessing...
>
> SoCalMike and Jim Beam have pointed out that, at least in
> some parts of the country, the gasoline formulation for
> winter is different, and of a lower calorific value, than in
> the warmer months. Various fed and state EPA web sites back
> this up, as well. So for some drivers, for sure this is a
> factor partly explaining their lower gas mileage around
> winter.

That's very cool, being familiar with subs - Not obnoxious at all.
I am not ex-NAVY. Just contracted for them at one of my previous
employers and have seen the inside of subs plenty of times.

By any chance, have you seen/used the digital panel meters, the ones
where a sub only needs to take few spares as one indicator holds
hundreds of different curves, are cloneable? That's my design.

I am sure you ar eright -- the gas formulation must be different and
must be affecting mileage.