Gas Tank Level Theory

Elle
snip

design.

Nah, I'm pretty dated. The last time I was in a submarine
engine room (as a civilian engineer) was in the 1980s.

 

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.
http://www.omega.com/techref/flowmetertutorial.html
MLD

 

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.
http://www.omega.com/techref/flowmetertutorial.html
MLD

 

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
exception).

 

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
exception).

 

Mine was around the late 80s/early 90s.
Pretty amazing machines them subs, imo.

 

Mine was around the late 80s/early 90s.
Pretty amazing machines them subs, imo.

 

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

 

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

 

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
faster.

Maybe the fuel guage reads slightly higher as the passenger compartment
warms up?

Mike

 

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
faster.

Maybe the fuel guage reads slightly higher as the passenger compartment
warms up?

Mike

 

(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

 

(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

 

efficiency.

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.

 

efficiency.

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
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..

 

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..

 

Waiving the right to remain silent, "Elle"

In summer and winter, you are buying different blends of gasoline.
That cold easily accont for the change in mileage.

Also... You should never use the fuel gauge to calculate mileage.

Fill up the tank and zero your trip odometer. Drive until you have
about 1/4 tank remaining. Fill up, then calculate mileage based on
actual miles driven. Do this several times and take an average.

 

Waiving the right to remain silent, "Elle"

In summer and winter, you are buying different blends of gasoline.
That cold easily accont for the change in mileage.

Also... You should never use the fuel gauge to calculate mileage.

Fill up the tank and zero your trip odometer. Drive until you have
about 1/4 tank remaining. Fill up, then calculate mileage based on
actual miles driven. Do this several times and take an average.

 

I agree, so far.

accuse it.

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.