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?
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
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.
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.
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.
Chances are they are not linear and probably gets worse with
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
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
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.
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...
use on subs
ridge - peaks and
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.
as a rule
The NAVY is
second - remember,
their tanks. The
drop a plumb
how bad an
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.
We'd get this
something to their
humidity of air
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
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.
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...
fifty years old
odometer and fill up
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
lie than disturb
procedure if that is
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.
Gas gauges are notoriously inaccurate when it comes to a smooth range of
motion regarding fill status. Most do exactly what you describe but
when they reach empty, they still have a gallon or two left. I had a
truck that when it was on empty, it was empty. I cannot remember the
times that I ended up having to walk down the road for gas because of my
preconceived notion of the "hidden" reserve.
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.
Probably the best article on the topic of "when to fill your gas tank" is
discussed in the link below. It's long but easy reading. All in all, since
the variance of below grade temperature is very low regardless of the
ambient temperature, it almost doesn't make any difference when you fill the
tank. Having said that--it's best not to do it right after the underground
tank has been filled when the fuel will most likely be at its warmest.
I read about this elsewhere a few weeks ago, and it seems
reasonable to me.
I am not particularly concerned about when I fill the tank.
(If only to minimize compulsiveness!) I do get great mileage
on this tiny 1.5 L engine, just about any time of year, with
just about any gas station (with only an occasional rare
It seems plausible. The volumetric coefficient of expansion of gasoline is
about 0.1% per degree C (http://tinyurl.com/bpp2k ) and the constant
circulation of the fuel through the engine compartment (through the fuel
pressure regulator) and warming by the in-tank pump could plausibly add 30
degrees C or more to cold fuel within an hour or two. That would expand 10
gallons to about 10.3 gallons, so if you've only burned a gallon or two in
that time it could appear as a noticable increase in fuel efficiency. If
there is less fuel in the tank, the same amount of heat will warm the fuel
Maybe the fuel guage reads slightly higher as the passenger compartment
(hey Mike - it has been a while since we've been in the same thread.
That expansion of gasoline is kind of interesting, if you think about
It really means that fuel should be sold by volume and temperature if
things were fair, huh?
There really should be an adjustment of price at the pump, taking tank
and outside temperature into account - a multiplier of the gas price.
We know the oil companies are not dishonest, (can't get my eyeballs to
roll down for some reason now :) but let's suppose they are totally
evil, it would be in their interest to heat the fuel before they sell
it to us.
If they heat it just one degree above ambient, it would be a totally
legal way of them making giving you 0.999 gallons for the price of 1
So in winter you're really getting ripped of a little, the buried gas
station tank being warmer than your car tank. By the time you've been
driving in the cold for a while, that gallon is really not a gallon
Just a thought - maybe a silly one but it does make me go
You guys better be happy I am on the right side of evil here :)
Thanks for the numbers. They are not as impressive as I
would have hoped, given the difference I seem to detect. I
believe I see around 30-50 miles difference to get from full
to a half-tank, from summer to winter. The roughly 0.3
gallons might account for maybe 12 miles of this. (My 91
Civic gets about 40 mpg and holds about 11+ gallons, though
these days I rarely let the car empty more than about 9.5
gallons between fillups.)
I am also not sure how or whether the non-uniform shape of
the tank figures into this.
I think it is a very interesting question you've asked. This is a great
One would imagine that all these errors accumulate (expansion, fuel
indicator) so maybe with all errors added in you'll see that 30-50
The only way to know for sure is to gather imperical, recording mileage
and gallons because you don't know if you can trust your gauge. It
would at least eliminate it as a variable and possibly accuse it.
Non-uniformity of the tank just tends to make the error worse as they
don't linearize the indicator very well. Of course, the tank shape is
the same in winter/summer. The way they approximate the linearity
probably walks with temperature, so there's most likely another small
Of course - not to give anyone mental whiplash - it could also be that
the car just runs richer in winter for whatever reason..
I think we're having a miscommunication. What I'm
challenging here is whether the gage reads more
proportionally to the actual weight of fuel in the tank in
the winter rather than summer. E.g. in winter, I get about
220 miles by the time the gage indicates half-full. In
summer, by contrast, I get about 270 miles.
worse as they
tank shape is
also be that
It's not something I'm losing sleep over. Just something
that might be worth mentioning when people come here
complaining that their fuel gage doesn't read in much
proportion to the fuel in the tank.
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