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Old 02-23-2012, 09:59 AM   #1
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Better gas mileage when tank is half full?? See my scenario

Alright. I'm new to the forum and joined specifically to state my experience and see if anybody could tell me why this happened.

I started a new job a month ago that's about 75 miles from my house and wanted to get a car that got better gas mileage then my Chrysler 300c. I ended up getting a 2000 Ford Escort ZX2. The drive is 95% interstate and I'm in North Carolina so the speed limit is 70 MPH the majority of the way. I normally have to fill up every other day and I am getting anywhere from 29 - 30 MPG on a consistant basis. I don't know for sure but I think the tank size is somewhere between 11 and 12 gallons. Anyway, one thing I noticed is when my tank was full, it would seem like I was getting very good gas mileage. According to the gauge, I was getting about 100 miles on a quarter tank but it declined greatly from there. I would get about 175 - 185 on half a tank, maybe 250 on 3/4 of a tank, and my gas light was coming on around the 300 mile mark. My gas gauge seems to be a little finicky so I never thought much of it. However, today I decided to fill my car up after about 215 miles and just a little under half full. I was getting 35 MPG and some change!!!! I'm glad but I do not know why. I was reading about gas evaporating and it evaporates more when the tank is less full however that is a drastic change. That is 5 more MPG and the only thing I changed was when I filled up my car. Does anybody have any explanation why this happened? Thanks in advance for the help.
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Old 02-23-2012, 10:33 AM   #2
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Re: Better gas mileage when tank is half full?? See my scenario

Gas gauges are neither linear nor accurate. When I've used 25% of my gauge, I've used about half of my tank.

That issue aside, the only vaguely accurate way to know your fuel economy is to fill up each time, don't try to top off after the pump clicks off, and divide miles by gallons. To be more accurate you can use the same pump, car facing the same direction, at the same time of day.

Even then there are too many uncontrolled variables to take a single fill too seriously, so a running average is the best way to know how well things are working for you. To that end I recommend starting a gaslog here, keeping one in a spreadsheet, or using another website's fuel log system.

If you've been filling up each time and getting 29-30 each time, you may have gotten a short fill this time. If so, it will become evident in the future. 16% is a lot to attribute to a change in evaporation but is easily attributed to a random short fill.

Knowing all that, I still get excited when I get a tank like that.
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Old 02-23-2012, 10:45 AM   #3
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Re: Better gas mileage when tank is half full?? See my scenario

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Originally Posted by theholycow View Post
Gas gauges are neither linear nor accurate. When I've used 25% of my gauge, I've used about half of my tank.

That issue aside, the only vaguely accurate way to know your fuel economy is to fill up each time, don't try to top off after the pump clicks off, and divide miles by gallons. To be more accurate you can use the same pump, car facing the same direction, at the same time of day.

Even then there are too many uncontrolled variables to take a single fill too seriously, so a running average is the best way to know how well things are working for you. To that end I recommend starting a gaslog here, keeping one in a spreadsheet, or using another website's fuel log system.

If you've been filling up each time and getting 29-30 each time, you may have gotten a short fill this time. If so, it will become evident in the future. 16% is a lot to attribute to a change in evaporation but is easily attributed to a random short fill.

Knowing all that, I still get excited when I get a tank like that.
Thanks for the reply.

I agree that gas gauges are inaccurate. That's why instead of assuming that my tank was burning gas slower when it was full, I actually put gas in it so I could see how much gas I had actually used. When I stopped at the pump, I knew according to the gauge, I should've put in between 6 and 7 gallons which remarkably is what I put in. I think it was 6.744 or something like that.

Just so you know, I got this reading with topping off my tank so I doubt it was a short fill. I always top off to the nearest dollar. I know that skews the MPG calculation but if it causes it to be off by .5 MPG, it's not a big deal to me. Anyway, I will post again tomorrow morning as I plan to fill up at the same station on my way to work. I'm actually pretty optimistic but would like to know the scientific reason behind this happening.

Thanks again.
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Old 02-23-2012, 11:33 AM   #4
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Re: Better gas mileage when tank is half full?? See my scenario

If you top off a half gallon on a 12 gallon fill, that will be 4%, or 1.2mpg. If you top off a half gallon on a 6 gallon fill, that would be 8%, or 2.4mpg. That's in addition to normal fluctuations in fuel economy and normal fluctuations in fill level - which also has twice the effect on a 6 gallon fill as on a 12 gallon fill.

A 6 gallon sample data does not produce a meaningful statistic and is very sensitive to minor inaccuracies. Even what I learn from 20 gallons isn't trustworthy.

With a 75 mile commute and 30-35mpg it won't take you long to burn through 50 or 100 gallons and get some decent data. For a couple weeks try running it down to E, for a couple weeks try filling every time you use up that first 1/4 mark.

I'm sorry to be such a downer, but it is what it is.
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Old 02-23-2012, 12:32 PM   #5
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Re: Better gas mileage when tank is half full?? See my scenario

mine is about the same way.

I usually hit 100-120 miles by the time it hits F (filling puts it past full)
250ish by the quarter mark
350ish by the half mark
475-500 on a tank.

those are summer numbers but you get the idea. we all deal with that.

I am a north carolinian myself. the last few days (minus monday) have been pretty warm so I have been seeing close to summertime numbers for fuel economy. that within itself can skew numbers. I doubt it has much to do with your evap system. it is a closed system so you really shouldn't lose much if anything.

start a gas log here. if you are paying with a credit card, just let the thing be when it stops by itself. it doesn't matter and you don't have to keep up with how much you spend. trust me, they'll send you an itemized bill.
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Old 02-23-2012, 01:38 PM   #6
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Re: Better gas mileage when tank is half full?? See my scenario

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Originally Posted by theholycow View Post
If you top off a half gallon on a 12 gallon fill, that will be 4%, or 1.2mpg. If you top off a half gallon on a 6 gallon fill, that would be 8%, or 2.4mpg. That's in addition to normal fluctuations in fuel economy and normal fluctuations in fill level - which also has twice the effect on a 6 gallon fill as on a 12 gallon fill.

A 6 gallon sample data does not produce a meaningful statistic and is very sensitive to minor inaccuracies. Even what I learn from 20 gallons isn't trustworthy.
Some of this is correct and some of it isn't. The topping off would actually make my MPG decrease and not increase, so if I don't top off a half a gallon on a 6 gallon fill, that would increase my MPG vs. if I would not have topped off.

Also, MPG changes you mentioned are not correct. The % you mentioned cannot be translated to MPG because you're missing a main ingredient........how many miles were actually driven. That number will obviously effect the difference in MPG between 6 gallons vs. 6.5 gallons.

I truly do appreciate your feedback. Like you, I am not trying to be a downer. I am just trying to get to figure this out. I will be sure to start up a log.

And Beef, thanks for your reply as well. Have you tried filling your tank up at different intervals to see if it effects your MPG?
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Old 02-23-2012, 04:16 PM   #7
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Re: Better gas mileage when tank is half full?? See my scenario

I have an MPG meter that tells me my average and instantaneous. mine is very consistant.

I would say that if you were going to really try to get better mileage, check into either a scangauge 2 ($170) or an ultragauge (~$85ish). google searches will lead you them. I am a big fan of the scangauge but I actually have both.

where abouts in NC are you from???
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Old 02-23-2012, 04:34 PM   #8
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Re: Better gas mileage when tank is half full?? See my scenario

As said, there's a reason the gas gauge is also called the "guess gauge"

Also, as far as fillup error goes, I top it off right up to where I can physically see the fuel. Obviously, as this CAN damage your evaporative emission control system, I can't recommend it. But it does serve to eliminate a large chunk of your fillup error. Choosing the same pump at the same gas station also helps to eliminate error. About the only thing you can't control pretty closely is the ambient temperature.
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Old 02-23-2012, 06:57 PM   #9
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Re: Better gas mileage when tank is half full?? See my scenario

You have to remember that to measure fuel in your tank, the gas gauge uses a float. This float has to be big enough to float, and when the tank is full, the float is actually submerged, since a full tank means no room above the gas for the float to sit. That means, for your first gallon or two of gas, your needle is still in the full position because your float is. Car manufacturers had two choice how to deal with this, show the tank as not all the way full when the float is at the top, or show it as full when it is slightly under. Instead of people trying to top off full tanks because their needle wasn't on full, car manufacturers decided the latter would be a better solution.

Also, like THC mentioned, they are not linear in nature either, they are a lever on a pivot which has a potentiometer embedded in it. On top of that, analog electronics generally operate in ranges of tolerance, unlike digital electronics, which give back precise values. With either of them, tolerances change with age, but since digital electronics are a more precise stepwise signal, changes in the equipment aren't as noticeable.
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Old 02-24-2012, 07:54 AM   #10
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Re: Better gas mileage when tank is half full?? See my scenario

Then the tank itself isn't an ideal shape. As the fuel level and float drop, the width and length of the tank can be varying. Changing the volume that each inch of range represents to the float gauge. Near the bottom, the fuel pump displaces volume the gauge sees as fuel space.

The gauge could be calibrated for that, but will cost more.
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