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I noticed While driving my car the fuel refills last me a lot longer when I go for the full tank over the half tank. I feel that it takes a lot longer to go through the upper half of the tank than the bottom half of the tank. I felt that keeping a full tank of gas is beneficial over the long run.

Does having air in the tank reduce the burning efficiency? Does keeping the tank full make the burning more efficient?.

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    Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Jul 9 at 19:13
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    that's very easy to measure: reset your distance counter when you get gas, then next time you get gas keep track of how far you drove, and how much fuel you used, do that for a few months to average things out, then compare the fuel efficiency to the amount of fuel purchased. (and in any case, that's a way of keeping an eye on the fuel efficiency of your engine, which should never change too much) – njzk2 Jul 10 at 22:12
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    The trouble your assumption that the fuel gauge decreases in direct proportion to the contents of the tank. It doesn't. – JLRishe Jul 11 at 11:42
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    When you fill up, reset your trip odometer. When you fill up next time, take the ratio of the distance you travelled vs the fuel you put in (using whatever units you understand) and that's your fuel efficiency. Do this every fill-up and you'll notice right away any time your fuel efficiency changes. – J... Jul 11 at 17:21
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    You didn't specify how you measured this, but you should be able to accurately measure your fuel efficiency by filling up to the same amount you filled up to last time and comparing how far you drove with how much fuel you refilled. The gauge not representing the true percentage filled shouldn't matter in this case (as long as it always shows the same value for the same amount filled). This would however still be subject to the inaccuracy from measuring using the gauge and variation in your fuel efficiency based on driving habits and environmental conditions (for which you can average results) – NotThatGuy Jul 11 at 18:21
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No. If anything you'll get worse gas mileage with a full tank because you're carrying a heavier load (more fuel = more weight).

What you are most likely experiencing is the top half of the tank is larger than the bottom half (in general terms). In other words, the sending unit (float level) in the gas tank most likely takes longer to traverse the top 1/2 then it does to traverse the bottom 1/2. This would make it appear you are getting better gas mileage on the top, because there is more gas in that top 1/2 than there is in the bottom.

EDIT: Criggie said in comments what I was trying to point out. When I say "top 1/2" and "bottom 1/2", I'm talking about what the gauge is reading. Not the actual 1/2 of the tank itself. This is in direct comment to what Fake Name stated about being nitpicky, as they are exactly right.

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    Nit picker in me wants to point out that if the "top half" contains more fuel then the "bottom half", they're not actually halves. – Fake Name Jul 10 at 4:05
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    @FakeName excellent spotting, you're right. But the gauge also doesn't necessarily represent the tank exactly, so 50% on the gauge could be a 60% full tank, etc. – Criggie Jul 10 at 6:52
  • Comments are not for extended discussion; this conversation has been moved to chat. – Pᴀᴜʟsᴛᴇʀ2 Jul 10 at 18:56
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The best way to determine the actual fuel consumption is to brim the tank, drive till nearly empty then brim the tank again.

Then you have an exact volume of fuel and a specific distance.

Repeat over three or four tanks and you will have an accurate fuel consumption figure based on your actual use.

The gauge is not linear in performance and not designed to be accurate - it is an indicator. Many gauges will gain or lose a bit on bends or going up, or down, hill. One thing that is done to hide this is to make the gauge slow to respond... Owners' manuals usually cover this by saying fill up at ¼ tank.

Also as pointed out in another answer the tanks are not designed with constant area through the depth of the tank. One reason is to avoid sloshing of fuel and picking up air; the other is that the space for the tank usually has to accommodate drive shafts or exhaust pipes etc.

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    Since the gauge isn't precise, how do you know when you're "nearly empty"? – Barmar Jul 10 at 10:51
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    @Barmar well, my car will deliberately misfire when fuel is too low, and that is after the yellow warning light has come on. If you still ignore that indication then it will shut down so as not to damage the pump, as if the pump runs without fuel it destroys itself. But you have to be really smart to get to that point. – Solar Mike Jul 10 at 11:03
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    @Barmar When you brim the tank, you know how much fuel have been used since the last time you brimmed it. It's not necessary to know when it's empty. It's enough being able to tell when it's full with some consistency. – Pere Jul 10 at 18:18
  • @Barmar - what's nearly empty? Half way across a desert is very different from it's at least two miles to the filling station. – Tim Jul 11 at 7:47
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    @Tim Optimist: glass is half full, pessimist: glass is half empty, Engineer: container is twice the needed size... – Solar Mike Jul 11 at 7:49
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IF - the tank was uniform in its dimensions - vertical sides, etc., and IF - the gauge sender worked exactly in a linear manner, and IF - the gauge responded in that same linear manner, and IF - the sender sent 'tank full' message to the gauge when it was full, then the half-full shown on the gauge would be truthful.

However, none of those can be true. The tank shape depends on its location, sharing space with other components. The gauge sender and gauge itself are rudimentary, electrically, and notoriously inaccurate. The float in the tank reaches its highest point before the tank is full. That last point is the red herring. The float stays in its highest position for longer, as it cannot drop lower until some fuel has been used.

So, it appears that the gauge becomes more pessimistic once the tank is only 'half full' - which in reality is often closer to 45% full.

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    My bet is that this is precisely what is going on. There is a significant amount of space in the tank and filler tube after the gauge hits full and before the OP stops filling their tank. So a significant amount of fuel has to be used before the gauge moves even a tiny bit off F. – David Schwartz Jul 11 at 2:45
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    As well as the gauge staying at the top for a while, some tank geometries can be extreme enough for noticeably slower points in the scale, e.g. they get wider for a bit. – Chris H Jul 12 at 13:17
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    Also, the float bottoms out before the tank is completely empty, so that's why the gauge doesn't accurately tell you how much fuel there is on the bottom end of the range either. Most cars can easily get another 30-50 miles after the empty warning triggers, though maybe you don't want to put that one to the test and get stranded on the side of the road. (There tends to be some gunk buildup at the bottom you don't really want in your engine anyhow, so best to fill up when it tells you to.) – Darrel Hoffman Jul 12 at 15:57
  • @DarrelHoffman - good point -the bottoming of the sender is as relevant as its 'topping'. – Tim Jul 12 at 16:56
  • I'd say that 45% at "half a tank" is generous. Also, when talking about a "full tank", don't forget about there being gas in the fill tube, going from the side of the vehicle to the tank itself. Then there's also likely sediment in the tank, which (depending on the amount) could prevent the float from hitting actual bottom or be clogging the pump intake when low on fuel. This clog could make it seem like the tank is empty when it's just full of junk. FYI, many brands of fuel system cleaners can address this sediment, but several don't and are just the same ethanol as in most gasolines. – computercarguy Jul 12 at 17:19
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You are being fooled by the psychology used by car manufacturers, who've discovered that A - if the gas indicator doesn't move for a while after a full tank, it makes the driver think the car is more economic than it is and B - if empty means empty, a lot of idiots will get themselves stranded.

Therefore most car fuel gauges these days leave ~50 km as a buffer after the gauge reads empty and 15-25km before the indicator moves from full on a full tank of gas. So the top half of your gas gauge isn't measuring 50÷, but e.g. 52.5÷ and the bottom 50÷ is measuring 47.5->5==42.5÷

EDIT

Sources:

https://thenewswheel.com/how-your-car-measures-gas-and-why-it-lies-to-you/ https://carrating.org/driving/fuel-gauge-lies-is-there-a-liar-on-your-dashboard

With modern technology, we have the ability to tell exactly how much gas is left, but we don’t. Engineer Phil Pierron, talking to Autoblog, said that customers, through surveys, have told automakers that we don’t actually want to run out of gas when they hit “E” — apparently, we like having the reserve so we can panic and drive gingerly to the closest gas station.

Similarly, it apparently makes us feel good to have the needle sit on “F” for longer. Partly this is because it gives us the illusion that we are either getting better gas mileage. However, it also makes you feel like you aren’t immediately burning through that tank of gas you just shelled out 40 bucks for (even though you totally are).

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  • Do you have any references for this? I'm thinking citations are needed for these claims. – computercarguy Jul 12 at 17:20
  • @computercarguy added – Eugene Jul 12 at 17:47
  • The CarRating.com article doesn't really count as a source, since it's more of an anecdotal article without any sources than their own experience and what they claim to be a story from someone who worked at an automaker. It makes for a great story, but it's lacking on any kind of real proof. – computercarguy Jul 12 at 19:08
  • @computercarguy Why not? Just because they quote an anonymous source? We've just had 4 years of most of the news about a US president coming from anonymous sources and generally being treated as credible(and cited on Stack Exchange) – Eugene Jul 12 at 23:36
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    @computercarguy - I don't think we require as strict referencing as Skeptics.SE here. The reference itself doesn't require 100% proof, as it's a fairly subjective comment anyway. Remember, you can use your votes to show your support or otherwise without needing to jump on someone for having a slightly loose source. – Rory Alsop Jul 14 at 8:40
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I feel that it takes a lot longer to go through the upper half of the tank than the bottom half of the tank.

Your feelings are valid so if it makes you feel better then continue doing so. That feeling is going vary from car to car and from person to person.

However, science would dictate that a heavier car causes worse fuel economy but whether or not you'll notice a savings of .2 miles per gallon is directly related to how you track that data and your consistency in doing so.


I felt that keeping a full tank of gas is beneficial over the long run.

Yes! Try not to let your tank go below 1/4 full or else your fuel pump could overheat. https://mechanics.stackexchange.com/a/61944/12029

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You can't get better MPG with a full tank. The engine simply cannot use gaseous fuel. It must be liquid or the injectors cannot correctly meter.

The top half is larger due to "Unusable Fuel"

Open up a modern fuel tank and you'll find an electric pump in there. It creates high-pressure fuel (300 KPA/45 PSI or so) to supply the fuel injectors. You'll also find some funny baffles around the pump inlet. These make it impossible to use the very last fuel in the tank. This is, in aviation terms, "Unusable Fuel".

That does a couple of things for you: it keeps crud and (heavier) water out of your fuel injectors and pump. The pump needs fuel to cool and lubricate; it'll be destroyed running dry so it makes sure there is fuel for that.

enter image description here

In this illustration, the green/blue border is the true 50% mark on the tank. But the brown (unusable) area impinges on the green (bottom half) area, making the upper half larger.

Of course, a real tank's shape will also be irregular due to needing to clear vehicle components. The empty/full gauge isn't sophisticated enough to account for these variations (or parking slope)... so on some cars that too will affect things.

They generally aim to have "E" be at the "unusuable fuel" line. There's a little bit of margin on unusable fuel due to splash and slosh: fuel can sometimes get over the baffles to a degree.

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It depends.

Despite the number of answers "confirming" no, I'm going to provide some outside sources that indeed show you're correct, or at least in some situations you can be - although it's not to discredit the points made, and the difference in reality is negligible, in either direction.

There are several factors that affect your gas mileage based on how full your gas tank is. This includes, but not limited to:

  • Size of the tank
  • Shape of the tank
  • Ambient temperature

See here, specifically point 2.:

  1. Gas Mileage

Some will argue that because the gas tank is lighter when its low, a low tank will give the car more fuel efficiency. A lighter load does require less gasoline, but the weight of a full fuel tank is not significant. A gallon of gasoline weighs only 6.3 lbs. (for reference milk weighs 8.6 lbs. per gallon), so even in most large vehicles the total weight of a full tank is under 250 lbs. Your car may actually be less efficient when the tank is near-empty, as more air in the tank can increase fuel evaporation.

And also this Quora answer:

In theory yes but, in practice it could actual hurt your mileage.

The reason is that the fuel will slosh and can get into a rhythm that actual makes your engine work harder. Ask any trucker or farmer that hauls liquid, a full tank is easier to move than a partly full tank that is sloshing.

I'm not going to go into the opposite point as it's been well discussed, and certainly carrying less weight does increase your gas mileage. But compared to the weight of a car, a half tank of gas is well... effectively nothing. Just like the loss from evaporation or the fuel sloshing about is effectively nothing.

I think the important thing is that each of these differences, whether in favor of better gas mileage with a full tank of a half tank, are negligible

It's kind of like driving around looking for a better price on gas, you'll waste enough gas looking you won't save any money - you're better off putting your attention elsewhere.

The first link I provided gives other reasons it is beneficial to keep a full tank, and I typically fall into this line of thinking, but again, in general it's not worth the concern either way. Just don't hit empty.

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  • DV care to explain why? Or just an angry Anthony that doesn't like logical explanations? – TCooper Jul 20 at 21:08
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Yes, Your car will operate normal with a full tank and not less. If you have less gas in the tank it takes more electrical energy to pump the gas to the front therefore there will be a quicker evap. and more gas used. Ever notice how long it takes for the gas gage to go down from full to half and quicker after a half tank.

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    The fuel tank is basically sealed so gas doesn't evaporate away (just into the air above the fuel, which is a relatively tiny amount). Leaving a car in a closed garage for days doesn't leave the air smelling like gasoline, or ready to explode if you lit a match. The fuel pump energy requirement is also negligible if it's a real effect, and probably more than balanced out by extra energy required to carry around the extra mass of fuel. The key effect is actually that the gas gauge isn't linear in mass or volume of fuel remaining, unlike a jetliner with a digital lbs / kg readout. – Peter Cordes Jul 11 at 0:02
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    Your last sentence makes sense, but the rest..? – Tim Jul 11 at 7:45
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    @Tim the last sentence is simply a restatement of the question. – manassehkatz-Moving 2 Codidact Jul 11 at 16:32
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    @manassehkatz-Moving2Codidact - obviously my point was made too subtly... – Tim Jul 11 at 17:24
  • While worded poorly, it's not wrong. It's also negligible, just like the counter arguments. – TCooper Jul 13 at 16:05

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