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?.
zeptal se 9. července 2021 ve 18:56
Anwar Elhadad Anwar Elhadad
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Vítejte na stránkách údržby a oprav motorových vozidel!
9. července 2021 ve 19:13
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)
10. července 2021 ve 22:12
The trouble your assumption that the fuel gauge decreases in direct proportion to the contents of the tank. It doesn’t.
11. července 2021 ve 11:42
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.
11. července 2021 ve 17:21
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)
11. července 2021 ve 18:21
8 odpovědi 8
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.
odpověděl 9 v 2021:19
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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.
10. července 2021 ve 4:05
@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.
10. července 2021 ve 6:52
Komentáře nejsou pro delší diskusi; tato konverzace byla přesunuta do chatu.
10. července 2021 ve 18:56
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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odpověděl 9 v 2021:21
Solar Mike Solar Mike
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Since the gauge isn’t precise, how do you know when you’re «nearly empty»?
10. července 2021 ve 10:51
@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 opravdu smart to get to that point.
10. července 2021 ve 11:03
@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.
10. července 2021 ve 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.
11. července 2021 ve 7:47
@Tim Optimist: glass is half full, pessimist: glass is half empty, Engineer: container is twice the needed size.
11. července 2021 ve 7:49
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 byl 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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odpověděl 10 v 2021:7
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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.
11. července 2021 ve 2:45
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.
12. července 2021 ve 13:17
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.)
12. července 2021 ve 15:57
@DarrelHoffman — good point -the bottoming of the sender is as relevant as its ‘topping’.
12. července 2021 ve 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.
12. července 2021 ve 17:19
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÷
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).
odpověděl 12 v 2021:5
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Do you have any references for this? I’m thinking citations are needed for these claims.
12. července 2021 ve 17:20
@computercarguy added
12. července 2021 ve 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.
12. července 2021 ve 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)
12. července 2021 ve 23:36
@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.
14. července 2021 ve 8:40
I cítit 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 cítit 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
odpověděl 12 v 2021:19
MonkeyZeus MonkeyZeus
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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.
Horní polovina 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.
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.
