# Gas Tank Level Theory

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• posted on January 12, 2006, 4:32 am
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?
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<%-name%>
• posted on January 12, 2006, 5:13 am
Elle wrote:

isnt there a potentiometer in the tank operating the gauge that could be kinda wonky?
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<%-name%>
• posted on January 12, 2006, 1:32 pm
SoCalMike wrote:

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
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<%-name%>
• posted on January 12, 2006, 3:48 pm

I couldn't rule that out, sure.

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.
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<%-name%>
• posted on January 12, 2006, 4:20 pm
Elle wrote:

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.

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...
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<%-name%>
• posted on January 12, 2006, 4:36 pm

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.

Good anecdote.

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.

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.
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<%-name%>
• posted on January 12, 2006, 4:56 pm
Elle wrote:

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.
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<%-name%>
• posted on January 12, 2006, 5:21 pm
Elle snip

Nah, I'm pretty dated. The last time I was in a submarine engine room (as a civilian engineer) was in the 1980s.
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<%-name%>
• posted on January 12, 2006, 6:21 pm
Elle wrote:

Mine was around the late 80s/early 90s. Pretty amazing machines them subs, imo.
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<%-name%>
• posted on January 12, 2006, 3:11 pm
Elle wrote:

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
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<%-name%>
• posted on January 12, 2006, 3:51 pm

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.

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.
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<%-name%>
• posted on January 12, 2006, 9:04 pm
Elle wrote:

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.
<G>
JT
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<%-name%>
• posted on January 12, 2006, 4:23 pm
Elle wrote:>

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.
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<%-name%>
• posted on January 12, 2006, 5:49 pm

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. http://www.omega.com/techref/flowmetertutorial.html MLD
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<%-name%>
• posted on January 12, 2006, 6:16 pm

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 exception).

:-)
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<%-name%>
• posted on January 12, 2006, 11:59 pm

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 faster.
Maybe the fuel guage reads slightly higher as the passenger compartment warms up?
Mike
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<%-name%>
• posted on January 13, 2006, 1:47 am
Michael Pardee wrote:

(hey Mike - it has been a while since we've been in the same thread. How's things?)
That expansion of gasoline is kind of interesting, if you think about it: 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 gallon. 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 anymore.
Just a thought - maybe a silly one but it does make me go "mmmmmmmmmmm..." You guys better be happy I am on the right side of evil here :)
Remco
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<%-name%>
• posted on January 13, 2006, 2:22 am
snip

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.
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<%-name%>
• posted on January 13, 2006, 2:46 am
Elle wrote:

I think it is a very interesting question you've asked. This is a great discussion.
One would imagine that all these errors accumulate (expansion, fuel indicator) so maybe with all errors added in you'll see that 30-50 miles discrepancy. 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 error.
Of course - not to give anyone mental whiplash - it could also be that the car just runs richer in winter for whatever reason..
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<%-name%>
• posted on January 13, 2006, 3:54 am
snip

I agree, so far.

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.
Roughly.

Right.
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.