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Discussion Starter · #1 ·
I just completed a 2400 mile trip with my Limited. No problems, very comfortable, plenty of power, good mpg. A curious thing resulted on the few occasions I filled up with gas when I had under 130 miles worth left in tank. Typically I would fill up sooner when the tank level was 1/2. Under those conditions the average mpg would at the time of fill up be the standard 23 - 24 mpg. However, when the tank was low as described above the mpg rating starting off from the refuel would show up as 17. Of course after some driving it would rise again. This was a consistant experience, I was just curious as to the cause. My guess is that it somehow has to do with the pressure in the tank. ???
 

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Based on my record and observation, the average MPG is very precise and it should repeat the same average MPG. The computer is very good at determining the MPG based on the measured volume of the gas tank and distance traveled (i.e. miles per gallon calculation).

I bet this is your problem: you reset the trip meter (Trip A or Trip B) every time you refuel, causing the computer to repopulate the data and determine the new average MPG.
 

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Discussion Starter · #3 ·
Yes I do a reset each time. That in of itself though would not explain the different mpg if I run the tank down to 1/4 versus 1/2. The experience is consistant.
 

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Maybe it seems just seems consistent, but the amount you have left in the tank at fillup might be a non-factor(?).

I think the average MPG right after a fillup and reset of the trip odometer begins to display after about a 1-2 miles of driving. For example, if I hop on the freeway after a fillup I'll see an average of high 20s. If I stay in the suburbs and make a few stops, it can be in the low teens. Eventually it all averages out more or less over the remainder of the tank.
 

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Yes I do a reset each time. That in of itself though would not explain the different mpg if I run the tank down to 1/4 versus 1/2. The experience is consistant.
everytime you hit the reset button, you lost all of your previous data.

Previous data:
-15mpg
-15mpg
-25mpg
-28mpg
-30mpg
-27mpg
-25mpg
average=23 - 24mpg

New data:
-17mpg
-17mpg
-17mpg
-17mpg
-17mpg
-17mpg
average= 17mpg

Had you not reset the trip meter, your average will be higher @ 20 - 21mpg.

I suggest you log about 3000miles to determine your average MPG of the vehicle to reflect your driving habit.
 

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Also you can try switching between trip A-B. I reset trip A with each fill up and trip B with every oil change. That way I can see the mpg for each tank as well as the mpg for each oil change. The tank is almost always lower because it starts with data that includes accelerating in town a lot as I leave the gas station.
 

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@packout, I think you're describing a Starting Value for the mpg computer. My old Mercedes had a starting value, where as soon as I pulled out from the gas station (after resetting the trip) I'd see "17.0" in the avg mpg display. Mercedes forces a starting value with some weighting (say, 10 miles worth of 17.0), I imagine so that the average mpg doesn't oscillate crazily like the instantaneous mph during the initial miles on that tank or reset, thus confusing the average driver (who thinks that averages should always move slowly).

But the Ascent doesn't seem to do this. Instead of a forced starting value and some fake weighted data to stabilize the average for a few miles, my Ascent shows "---" for several miles, and then finally throws a number in there. Since it waited throughn some miles, this average mpg number is pretty stable (I guess based on average mpg over those xx miles) and doesn't move wildly like the instant figure.

So I think that you are just seeing an average that is really calculated, and not a fake starter. Maybe you filled up at city stations for your lower mileage fills, and at a highway station for the others, so your pullaway first xx miles showed as distinctly different mpg's?

F.S.
 

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Discussion Starter · #8 ·
everytime you hit the reset button, you lost all of your previous data.

Previous data:
-15mpg
-15mpg
-25mpg
-28mpg
-30mpg
-27mpg
-25mpg
average=23 - 24mpg

New data:
-17mpg
-17mpg
-17mpg
-17mpg
-17mpg
-17mpg
average= 17mpg

Had you not reset the trip meter, your average will be higher @ 20 - 21mpg.

I suggest you log about 3000miles to determine your average MPG of the vehicle to reflect your driving habit.
I maintain a spreadsheet of all actual fueling fill ups, costs and mileage. I had been using the digital readouts as a means of keeping a closer eye on any irregularities that might be occurring. I think you are correct that it takes some time to develop the data for average mpg due to the reset but it still does not address the issue of why the difference exists only when the fuel tank is low. There is no difference with city versus highway time of fills. This has occured on the highway during the same trip so all other factors were the same. The only difference was the amount of fuel left in the tank. It does not impact anything and the actual mileage is not impacted. I was just wondering what it was about the fuel tank interactions that appears to be changing the initial average mpg.
 

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I just completed a 2400 mile trip with my Limited. No problems, very comfortable, plenty of power, good mpg. A curious thing resulted on the few occasions I filled up with gas when I had under 130 miles worth left in tank. Typically I would fill up sooner when the tank level was 1/2. Under those conditions the average mpg would at the time of fill up be the standard 23 - 24 mpg. However, when the tank was low as described above the mpg rating starting off from the refuel would show up as 17. Of course after some driving it would rise again. This was a consistant experience, I was just curious as to the cause. My guess is that it somehow has to do with the pressure in the tank. ???
Do you idle your car while filling up your gas tank?
 

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All my cars start out low when I reset the mpg tracker (unless I reset it after getting underway). On my Kias I can reset mpg separately from tripmeter. I can also choose whether or not I want to reset mpg when I fill the tank. I have that setting enabled, so it resets every fill-up. Whenever it's reset, I usually have to drive a mile or two before the gauge tells me my estimated mpg.
Like others have said, if you reset at the gas station, your mpg includes navigating out of the gas station, accelerating from a stop, plus whatever traffic lights you encounter, so it will show a lower value.
 

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This may not be what you are talking about, but the average mpg adjusts to the current driving conditions. So does the expected remaining range. I reset trip meter A everytime I fill but never reset B.

I just got my Ascent out of the body shop (storm damage repair). I had filled it shortly before taking it in. When I got it, it showed the average mpg as 10.5 and 130 miles left. After a day of normal driving and adding 60 miles, it shows an average mpg as 14.1 and 220 miles left. I expect both numbers to increase before my next fill up.
 

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Follow up to this topic.
  1. I've been alternating using my B trip and A trip as my "tank's running MPG", and using the other one for more localized MPG readings. We all knew this but I wanted to make sure - there is no difference in how A and B function.
  2. When I reset one of the trips, the car waits through exactly 1.0 miles on the trip odometer before it goes from showing "--" to displaying an Average MPG for the trip being measured. Every time, 1.0 miles.
  3. From the different Average MPG's that pop up at 1.0 miles, I conclude that my Ascent is calculating Average MPG throughout the first mile, even though it is not diaplaying it. The number always made sense to what kind of driving I was doing at the first mile after reset. I also changed when I reset, like filling up empty tanks, filling up half full tanks, and filling up 3/4 tanks. All of the numbers made sense and did not seem to correlate low or high to previous tank level.
These are just observations on how mine works, I can't conclude anything about Packout's observation after fill-ups -- that's odd stuff.

F.S.
 

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When I reset one of the trips, the car waits through exactly 1.0 miles on the trip odometer before it goes from showing "--" to displaying an Average MPG for the trip being measured. Every time, 1.0 miles.
This is how my other two Subies (2012 and 2014 Outbacks) have performed the average MPG calculations after a reset on either/both the trip counter. It makes total sense to stabilize the display as the first mile would be as dizzying to look at as the instant MPG display.

Every so often, I do my own MPG calculations (miles driven since last fuel-up / # gallons to full) and the Subaru Average MPG computer calculations have consistently been 1 to 1.5 MPG optimistic. Nothing I have done (either reseting the trip computer or disconnecting the battery for whatever reason) affected or the way the car calculations the average MPG as others have stated in this thread.
 
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My favorite trick was filling up on the way into work. It was 8 miles from the gas station to the office. It's all slightly downhill. If I catch all the lights, I could register 46mpg in my Ascent. One time in my Outback, I got 64mpg.

Conversely coming home from Flagstaff, there's a gas station right before a huge climb out of the valley. I'd have 10-12mpg after climbing out of there, then it would come up as I coasted down the other side :D
 
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FWIW: Page 200 in my manual says:

NOTE
When the trip meter display is reset, the
average fuel consumption corresponding
to that trip meter display is not
shown until the vehicle has subsequently
covered a distance of 1 mile
(or 1 km).
 
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