# Gas Tank
**URL:** https://boards.straightdope.com/t/gas-tank/9463
**Category:** Factual Questions
**Created:** [March 28, 2000, 5:14pm UTC](https://boards.straightdope.com/t/gas-tank/9463 "2000-03-28T17:14:00Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@malaka](https://boards.straightdope.com/u/malaka)
#### Post date: [March 28, 2000, 5:14pm UTC](https://boards.straightdope.com/t/gas-tank/9463/1 "2000-03-28T17:14:00Z")
I own a 94 Toyota Tercel. It has a 10 gallon tank. I have pushed the envelope and managed to glide into the gas station showing 300 miles to the tank. However, I’ve also noticed that, when my trip odometer shows around 200 miles, often my gas gauge will still show a half tank available. However, this second half of gas will always run out around 300 miles.
Why does the first half of the tank seem to last 2wice as long as the second? My driving habits are veried throught the entire tank (freeway and city driving), so I don’t think that could be the cause.
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### Author:  [@SingleDad](https://boards.straightdope.com/u/SingleDad)
#### Post date: [March 28, 2000, 5:17pm UTC](https://boards.straightdope.com/t/gas-tank/9463/2 "2000-03-28T17:17:00Z")
A gas gauge is not a precision instrument. Depending on the design of the gauge and the shape of the tank, there usually will _not_ be a linear relationship between the reading of the gauge and the amount of gas.
I’ll do some more research and see if I can find the actual design of a real gas gauge.
* * *
If Cecil Adams did not exist, we would be obliged to create Him.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [March 28, 2000, 5:24pm UTC](https://boards.straightdope.com/t/gas-tank/9463/3 "2000-03-28T17:24:00Z")
An accurate fuel gauge is a lot more expensive than a simple device that measures the level of fuel in an irregularly shaped tank. I think some aircraft measure capacitance of the fuel to the tank to calculate quantity.
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### Author:  [@SingleDad](https://boards.straightdope.com/u/SingleDad)
#### Post date: [March 28, 2000, 5:25pm UTC](https://boards.straightdope.com/t/gas-tank/9463/4 "2000-03-28T17:25:00Z")
From [Click & Clack](http://cartalk.cars.com/Info/Cyberchumps/Questions/q170.html):
> [@](#):
>
> The sending unit {of the gas gauge} is nothing more than a rheostat–a variable resistor. As the float moves up and down in the tank the resistance changes, which varies the reading on the gauge.
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### Author:  [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)
#### Post date: [March 28, 2000, 5:29pm UTC](https://boards.straightdope.com/t/gas-tank/9463/5 "2000-03-28T17:29:00Z")
It’s been pretty consistent with me in the 20+ years I’ve been driving.
In every vehicle I’ve ever used, the first half significantly bigger than the second half.
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### Author:  [@billehunt](https://boards.straightdope.com/u/billehunt)
#### Post date: [March 28, 2000, 9:29pm UTC](https://boards.straightdope.com/t/gas-tank/9463/6 "2000-03-28T21:29:00Z")
**SingleDad** wrote **Click & Clack** wrote
> [@](#):
>
> The sending unit {of the gas gauge} is nothing more than a rheostat–a variable resistor. As the float moves up and down in the tank the resistance changes, which varies the reading on the gauge.
Yeah. In the '63 Volkswagon squareback I had as a teenager, the guage was an aluminum tube-shaped thing maybe 1" across and 6" tall that sat inside the gas tank, mounted on the top. Inside the tube was a float that moved up and down floating on the gas which was attached to a rheostat mounted along the inside side of the tube.
The odds are, of course, that none of the components (gas tank, rheostat, and indicator) are purely linear. So, there are plenty of ways for the system to be inaccurate.
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### Author:  [@Delta-9](https://boards.straightdope.com/u/Delta-9)
#### Post date: [March 28, 2000, 9:35pm UTC](https://boards.straightdope.com/t/gas-tank/9463/7 "2000-03-28T21:35:00Z")
I own a '91 Plymouth 6000 and the gas gauge does the same thing as the OP. Stays at full for a long time, but once it hits 1/2 tank it drops like a rock.
I thought maybe it was engineered that way to create a sense of urgency, so you wouldn’t wait too long to fill up again.
* * *
Eagles may soar, but weasels don’t get sucked into jet engines.
[Delta-9 Home Page](http://www.delta9studio.com)
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### Author:  [@Nurlman](https://boards.straightdope.com/u/Nurlman)
#### Post date: [March 28, 2000, 10:36pm UTC](https://boards.straightdope.com/t/gas-tank/9463/8 "2000-03-28T22:36:00Z")
Don’t forget that the little “E” and “F” aren’t always the physical limits of the gauge.
If you top off the tank just a little when you fill up, you’ll find that the needle goes **above** “F,” thus increasing the number of miles you’ll get out of the first “half” of the tank.
And since the “E” is often at the top of a red range, or is purposefully placed in such a way that the needle shows “E” when you actually still have 3/4 of a gallon left. (As it was in my old Honda.)
Just using that example, 3/4 of a gallon in a 10 gallon tank means that you’ll get 7.5% more mileage out of the top half than you do the bottom. Add to that the extra float you get above “F” from filling up, and you’ve accounted for a pretty fair chunk of your “missing mileage.”
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### Author:  [@Oblio](https://boards.straightdope.com/u/Oblio)
#### Post date: [March 28, 2000, 11:17pm UTC](https://boards.straightdope.com/t/gas-tank/9463/9 "2000-03-28T23:17:00Z")
The sending unit on my Cessna 172 is manufactured by Stewart Warner. It uses a brass float attached via an arm to a variable resistor. One reason for the non-linearity may be that the float will remain at the full position and submerged in fuel until the fuel level drops down a bit. If the top of the float is normally 3/4 inch above the fuel then 3/4 inch of fuel must be burned before the gauge will move off of full. I’m not sure if this would explain the 1st half vs. 2nd half discrepency, but it does explain the stuck on full phenomenon I have seen in cars and aircraft after filling up.
* * *
A point in every direction is like no point at all
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### Author:  [@Oblio](https://boards.straightdope.com/u/Oblio)
#### Post date: [March 28, 2000, 11:25pm UTC](https://boards.straightdope.com/t/gas-tank/9463/10 "2000-03-28T23:25:00Z")
Re-reading Nurlman’s post above reminded me that in my above example there would be 1/4 inch of fuel left when the gauge was bottomed out and presumably on E.
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### Author:  [@LBJay](https://boards.straightdope.com/u/LBJay)
#### Post date: [March 29, 2000, 1:17am UTC](https://boards.straightdope.com/t/gas-tank/9463/11 "2000-03-29T01:17:00Z")
I had a '59 Beetle. That was the last year VW’s were built with no gas gauge, whatsoever. It had a teeny little reserve tank which consisted of a tray built into the main tank. When you would fill up the tank, this tray would hold onto about a 1/4 gallon of gas, if that much. You would drive until you ran out of fuel, reach down to twist a lever on the floor that would turn the tray over spilling the remaining gas into the tank. You could then re-boot the bug and hopefully drive to the nearest gas station. Tell me again why we have such fond memories of VW Beetles.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [March 29, 2000, 8:42am UTC](https://boards.straightdope.com/t/gas-tank/9463/12 "2000-03-29T08:42:00Z")
Actually, Click and Clack answered (well, to the extent they answer anything) this exact question. Take a look at [http://cartalk.cars.com/Columns/CC/CC7543TXT.html](http://cartalk.cars.com/Columns/CC/CC7543TXT.html) .
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### Author:  [@neutron\_star](https://boards.straightdope.com/u/neutron_star)
#### Post date: [March 29, 2000, 8:51am UTC](https://boards.straightdope.com/t/gas-tank/9463/13 "2000-03-29T08:51:00Z")
'92 Tercel, same experiences as the OP. I hate to “me too,” but figured I might as well since I have the same make and model as the OP.
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### Author:  [@sailor](https://boards.straightdope.com/u/sailor)
#### Post date: [March 29, 2000, 11:29pm UTC](https://boards.straightdope.com/t/gas-tank/9463/14 "2000-03-29T23:29:00Z")
I have also noticed this in every car I have owned and I find it useful as the gauge gives you greater resolution where you want it, ie. when you are running low on gas.
i am not even saying it was designed that way, it may be a flaw, but a useful flaw.
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### Author:  [@manhattan](https://boards.straightdope.com/u/manhattan)
#### Post date: [March 30, 2000, 1:11am UTC](https://boards.straightdope.com/t/gas-tank/9463/15 "2000-03-30T01:11:00Z")
::This is a test post. Please ignore it::
::If you see multiposts above, please ignore them, too. I’ll clean them up::
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### Author:  [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)
#### Post date: [March 30, 2000, 8:25am UTC](https://boards.straightdope.com/t/gas-tank/9463/16 "2000-03-30T08:25:00Z")
Another reason would be that the gas tank will have a sloped bottom or will be tilted so that the fuel outlet will be at the lowest point of the tank. That means the the tank will not have a linear relationship between the level of the gas and the amount remaining, and this will get more pronounced as you get near the end.
My Grumman AA1 had very simple gas gauges - A calibrated tube inside the cockpit, connected with a hose to the gas tank. Since fluids that are connected will always seek the same level, the tubes would show you exactly how much gas was in the tank.
In theory. In practice, they were useless. If a fuel filler cap had a slight seal problem, rushing air could pressurize the tank and force the gas level in the tube higher than it should have been. If the airplane wasn’t in perfect coordinated flight acceleration forces would make one tank read high and another low, and if the wings weren’t perfectly level they would read incorrectly as well.
It hardly matters, since no pilot worth his salt will rely on gas gauge readings. I’m not sure I ever looked at the things other than for comfort reasons. You calculate your fuel burn, you know how much gas you pumped in, so you know how much is left. Who needs gauges? In a car it’s not that easy, since the mileage you get can vary a lot. In an airplane, it’s quite constant.
The best fuel gauge setup for a car would be a totalizer that calculate how much fuel has moved through the lines. Fill up the car, zero the totalizer, and you’ll know within a few tenths of a gallon exactly how much gas is left in the car. Larger airplanes use fuel totalizers (a little mechanical or solid-state device that measure fuel flow).
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### Author:  [@Danielinthewolvesden](https://boards.straightdope.com/u/Danielinthewolvesden)
#### Post date: [March 30, 2000, 9:34am UTC](https://boards.straightdope.com/t/gas-tank/9463/17 "2000-03-30T09:34:00Z")
I believe the above posting have got it. However, it is NOT good to “use the last few drops” in your tank. The bottom of your gas tank is usu full of icky stuff you don not want your car (attempting) to burn.
If you do do this, then get a bottle of those gasline/carb cleaners & run it thru, or check your fuel filter, or both.
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### Author:  [@handy](https://boards.straightdope.com/u/handy)
#### Post date: [March 30, 2000, 3:56pm UTC](https://boards.straightdope.com/t/gas-tank/9463/18 "2000-03-30T15:56:00Z")
Feldman answered this in one of his books.
Also, when you have no gas & it doesn’t show empty, the needle needs to be adjusted. I did that on a vw.
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### Author:  [@billehunt](https://boards.straightdope.com/u/billehunt)
#### Post date: [March 30, 2000, 5:58pm UTC](https://boards.straightdope.com/t/gas-tank/9463/19 "2000-03-30T17:58:00Z")
**Danielinthewolvesden** wrote
> [@](#):
>
> However, it is NOT good to “use the last few drops” in your tank. The bottom of your gas tank is full of icky stuff you do not want your car (attempting) to burn.
I’ve heard several people say this, and I think it’s hogwash. After all, the outlet from the gastank is on the bottom of the tank, so if there is crap in your gastank, just the act of filling the tank should get it swirling around and into the engine it goes.
As I say, I **think** it’s hogwash, but I have no proof. Anyone know for sure?
Also, for those that haven’t read **scr4** ’s passage from **Click & Clack** , it’s very good.
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### Author:  [@billehunt](https://boards.straightdope.com/u/billehunt)
#### Post date: [March 31, 2000, 6:08am UTC](https://boards.straightdope.com/t/gas-tank/9463/20 "2000-03-31T06:08:00Z")
By the way, this “using the last drops of gas” thing isn’t hypothetical for me. I’m a JIT kinda guy, and I rationalize that if I can go from as full as possible to as empty as possible per tankful, the fewer times I have to fill up over a year, and the more time I have in my life for non-fillup activities. The downside is that I’m a poor estimator of empty and I run out of gas maybe every four months. I carry a portable gas tank in the trunk, so I don’t get stuck.
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