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!!!! :eek: 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.

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. ;)

Re: Better gas mileage when tank is half full?? See my scenario
 

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.

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.

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.

Re: Better gas mileage when tank is half full?? See my scenario
 

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?

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???

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" :D

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.

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.

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.

Re: Better gas mileage when tank is half full?? See my scenario
 

Thanks guys again for the responses. I'm not disagreeing with the function, calibration, reliability, etc or the gas gauge. I made an observation that based off of the gas gauge, I appear to get better gas mileage when my tank is fuller vs. it being empty. My first two tests of that theory appear to support that. I was 33.50 MPG this morning when filling up with my tank being about 5/8 full. My question is is there any logical reason for this to happen? I'm going to keep tracking it just to see.

BEEF, I live in the Raleigh area and commute to Rocky Mount.

Re: Better gas mileage when tank is half full?? See my scenario
 

I'd be interested to see a gaslog showing it working over the course of 100 gallons or more. I cannot think of a logical reason for it to help.

Re: Better gas mileage when tank is half full?? See my scenario
 

shrem, I live in reidsville and commute to greensboro.

I still think that it is dependent on other factors. it has been abnormally hot this week (minus the weird snow last sunday night). we have seen 60s and 70s. I think if you were to run an entire tank again, you would probably see similar mileage. realize I am assuming on that one.

also, if you like this job, you may want to consider relocating. 75 miles is a lot.

I would say to start a gas log. they are handy little things and tell you a lot about trends over time.

Re: Better gas mileage when tank is half full?? See my scenario
 

75 miles is a lot. My wife and I have considered relocating but we're not quite sure. There's a lot of variables that go into that. There are a lot of people that commute from Raleigh so I'm hoping to maybe be able to car pool.

I will do a gas log and share it with everybody. I'm sure I can find an app on my phone. I will keep everybody up to date.

Re: Better gas mileage when tank is half full?? See my scenario
 

It seems to me that one would possibly get better gas mileage the more empty the tank is. Simply put, if all conditions are exactly the same, the tank would be less full and thus weigh less. Now, I don't know how much gasoline weghs by the gallon, but it stands to reason that your first gallon on a 14-gallon tank would not perform as well as your last gallon simply because there would be 13 less gallons of fuel and weight to transport. Makes sense to me, but would this much weight reduction for the final gallon or two even be worth 1 extra mpg?

Re: Better gas mileage when tank is half full?? See my scenario
 

I noted getting better fuel economy when under half a tank driving the Prius. There were likely other factors at work than just weight differences. The gen 2 Prius had a fuel bladder. When full and expanded, the bladder could be applying pressure to the fuel. This extra pressure could squeeze a little bit of extra fuel in whenever the pump turned on and/or the injectors opened. More than what the system called for.

While I don't know of any other car with a fuel bladder, there is column pressure. The weight of the fuel at the top of a full tank compresses the fuel down by the pump pick up. Whatever the cause of the pressure, if it had an effect, it likely isn't noticable until 55+ mpg.

IIRC, a gallon of gas is 6.4 pounds.

Re: Better gas mileage when tank is half full?? See my scenario
 

I thought of the pressure thing, but the computer will adjust for that based on O2 sensor readings...or it'll throw a code.

The weight issue would matter to weight-sensitive cars but I don't believe most cars on US roads are sensitive to weight fluctuations less than 15% or so.

Re: Better gas mileage when tank is half full?? See my scenario
 

While the sensors involved in running the car are more accurate than the tank gauge, they do have a tolerance range before throwing a code in which extra fuel can squeeze by. Tiny amounts that won't be noticed in lower mpg vehicles if it is happening.

The other factor in play is driving style. Unless the computer learns, and takes the tank level factor into account as soon as the pedal is depressed, pulse and glide may amplify the effects of the tank pressure. With more times going from off to on with accelerator, there is more chances for the pressure squeezed gas to slip by before the computer can adjust. It's miniscule amounts at each event, but P&G allows more of them to add up over a tank.

Re: Better gas mileage when tank is half full?? See my scenario
 

Alright guys. Here's my update. It took me a little longer for various reasons but the data is accurate. Just in case anyone is wondering, I filled up at the same gas station (not always the same pump) and did not top off. I use this car to drive to and from work only and I take the same route every time. There shouldn't be any variability. I deleted the data for days when I knew I drove the car differently ie. running a lot of errands after work. The data is the date, odomoter reading when I filled up, amount in gallons put into the tank, and the MPG.

3/6 - 153872 - 9.42 - initial fill up
3/12 - 154140 - 8.77 - 30.63
3/14 - 154382 - 7.53 - 32.13
3/18 - 154708 - 10.07 - 32.4
3/19 - 154862 - 4.16 - 37.04
3/23 - 155170 - 9.97 - 30.83
3/27 - 155324 - 5.45 - 28.27
3/28 - 155479 - 4.89 - 31.74
3/31 - 155697 - 6.93 - 31.49

As you can see, I was a happy man on the 19th but couldn't duplicate those numbers. That seems like a pretty big jump so I don't know what happened. I have done some work on my car here recently. I kept getting a P0125 fault if I'm not mistaken which deals with the coolant system. I replaced the coolant temp sensor because I knew that was messed up. I got the error 2 more times so I replaced the thermostat this weekend. Based off of how my temperature gauge was reading, I think it was stuck open or coolant was getting through the gasket or something. The temp reading is much more consistent now. Besides something related to that, I don't know what would have caused my MPG to jump so drastically. :-?

Re: Better gas mileage when tank is half full?? See my scenario
 

What was the weather like on those days?