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Ralstorm
10 XT Auto
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66 Posts
Discussion Starter #1
So my ever so spiffy 2010 XT's ave MPG reading is off about a 1 MPG. It tells me that I have earned 22.3MPG but when I do the math its closer to 21.0MPG. Does anyone know if you can recalibrate it?
-Anthony
 

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Premium Member
2015 XC diesel CVT
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4,055 Posts
No calibration is possible, it's only an indication (just like every other in-dash mpg display...)
 

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2009 Forester X 4 A/T
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305 Posts
All the average MPG displays seem to be a bit optimistic. My '09 reads 1.5 MPG high on a consistent basis. The one in my old Buick was really off- showing 3 to 5 mpg better than reality. None are adjustable.
 

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2007 Forester XT manual
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1,452 Posts
So what's new? Subaru likes to build them with inaccurate speedos so why not the fuel consumption readout too. Have to be consistent. :icon_rolleyes:
 

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2014 2.0XT CVT
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3,069 Posts
how do you know it's off and does 1mpg 'average' really matter? The problem you'll hit is 1)did you fill up the tank exactly the same every single time? 2)did you use the same exact pump under the same exact conditions each fill up? 3)were your tires inflated exactly the same each time resulting in identical mileage readings on the odometer each time? 4)did you use identical gas each time?
 

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Ralstorm
10 XT Auto
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66 Posts
Discussion Starter #8
how do you know it's off and does 1mpg 'average' really matter? The problem you'll hit is 1)did you fill up the tank exactly the same every single time? 2)did you use the same exact pump under the same exact conditions each fill up? 3)were your tires inflated exactly the same each time resulting in identical mileage readings on the odometer each time? 4)did you use identical gas each time?
All that shouldn’t matter. Miles driven/gallons filled = MPG. I am not saying it is off from one tank to the next but the HUD display of the MPG. HUD says 22.5MPG calculator says 21.0MPG. Yar, it does matter, at least to me :biggrin:
Ohh well. Thank you for ya'lls responces. Guess I'll just subtract 1.5 from what ever its telling me. The reason I ask is that one of my friends just got a 2010 Outback and his is accurate.
 

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2014 2.0XT CVT
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3,069 Posts
what are you using to calculate your miles driven and amount of gas used? If you fill your tank up and the pump automatically shuts off with say 99% of the tank capacity filled then you fill up again and that pump shuts off with 97% of the tank capacity filled then you get an inaccurate reading on gallons used. So if you start out at 16 gallons, you run it down to E then fill back up to 15.8 gallons your next trip you start out at 15.8 then run it to E then fill back up to 16 gallons, you now think you used 16 but you really used 15.8. (simplified as best I could) There are inaccuracies in your pump and in your odometer.
 

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2017 Foz 2.5i CVT
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what are you using to calculate your miles driven and amount of gas used? If you fill your tank up and the pump automatically shuts off with say 99% of the tank capacity filled then you fill up again and that pump shuts off with 97% of the tank capacity filled then you get an inaccurate reading on gallons used. So if you start out at 16 gallons, you run it down to E then fill back up to 15.8 gallons your next trip you start out at 15.8 then run it to E then fill back up to 16 gallons, you now think you used 16 but you really used 15.8. (simplified as best I could) There are inaccuracies in your pump and in your odometer.
I absolutely agree. I think I checked my calculated mileage vs the dash readout about twice (in about 29k miles of ownership). The first time I found the dash readout to be high by ~1 mpg, I think. Next time, thousands of miles later, it was spot on. I don't know why...

I would check the calculated mileage over 2-3 tanks to eliminate inconsistencies in gas fill, etc. Also, how do you know your gas station is giving you full gallons? I have no idea how much variance there is (in real life, not regulations) in the amount of fuel that gasoline pumps call a gallon. Is it 1%, or 10%?

There are a lot of variables here. In the end, your dash reading may be overstating your mileage, but maybe not.

George
 

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2010 Forester
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70 Posts
Because we don't drive in a laboratory, we can't reach an |absolute| value for MPG. Your dash reading is valuable for providing data within its own measurements. Knowing variances over differing driving conditions is more important than knowing which method achieves the more accurate result, anyway. At least outside of the lab.
 

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2009 Forester 2.5x
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702 Posts
My dash mpg readout has been optimistic by 1 mpg since my first fillup. I'm pleased that it has been so consistent.
Steve
09 2.5X AT
 

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2010 FXT Limited 4EAT Sport Shift
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1,461 Posts
Subaru's addition of electronic fuel usage ESTIMATORS has enlivened a once busy topic, now archived I suppose, but available by search.
Bottom line? The closest 'non-lab' assessment of fuel usage comes from an approach documented probably five years ago (in the context of this forum) by Peaty, who kept meticulous records of date, number of gallons, number of miles, and maybe comments on usage, weather, etc.
The more consecutive times you do this, the closer you come to VERY ACCURATE FUEL CONSUMPTION MEASUREMENTS. Those differences between fuel stop 'click' differences, get averaged out over time.

The new electronic fuel consumption guaging devices get you close enough to keep you from lying (well rounding up, or exagerrating), and for that, I support their continued use...;>)

-Quick
 

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2017 Foz 2.5i CVT
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My dash mpg readout has been optimistic by 1 mpg since my first fillup. I'm pleased that it has been so consistent.
Steve
09 2.5X AT
And wouldn't be all be more bummed if the mileage read 1 mpg low instead? :)

My wife doesn't like driving the Foz to work because she feels self conscious of the gas mileage (especially in comparison to her '07 Civic which gets 35 mpg) on her 43-mile (one way) commute.

George
 

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Ralstorm
10 XT Auto
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66 Posts
Discussion Starter #15
Ah I understand a lil better now. At first I didn’t get how any of those other factors uh... factored in. I think one other issue with the HUD MPG is that it only changes by 0.3 MPG i.e. 22.5 then would jump to 22.8, 23.1, etc. My guess it would be too much of a pain to have it read every 0.1. I would rather have it read under than over, at least that way I would get a happy surprise when I do it my flawed calculator way instead of a sad trombone sound Waaa Waaa Waaaaaaa.
 

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2009 Forester X
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Ah I understand a lil better now. At first I didn’t get how any of those other factors uh... factored in. I think one other issue with the HUD MPG is that it only changes by 0.3 MPG i.e. 22.5 then would jump to 22.8, 23.1, etc. My guess it would be too much of a pain to have it read every 0.1. I would rather have it read under than over, at least that way I would get a happy surprise when I do it my flawed calculator way instead of a sad trombone sound Waaa Waaa Waaaaaaa.
As an engineer who does lots of data analysis, this fact annoys the living hell out of me. It makes no sense to me for it to move in increments of 0.3. But oh well, can't win everything.

Also, assuming I'm right on how the car calculates the dash MPG, it's pretty simply statistics on why your calculated value will (almost) never match the dash reading. The dash MPG is most likely calculated on a moving average. I would guess that it calculates the MPG based on the fuel consumption over the last 100 miles (or conversely, the miles driven over the last 5 gallons consumed).

So, when you fill your gas tank up and calculate an MPG, it will be different than the reading on your dash because you're calculating the MPG based on an entire tank of gas (300 miles or so) and the car is basing it off of the last 100 miles. That is, unless your driving conditions are amazingly consistent.
 

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2017 Foz 2.5i CVT
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As an engineer who does lots of data analysis, this fact annoys the living hell out of me. It makes no sense to me for it to move in increments of 0.3. But oh well, can't win everything.

Also, assuming I'm right on how the car calculates the dash MPG, it's pretty simply statistics on why your calculated value will (almost) never match the dash reading. The dash MPG is most likely calculated on a moving average. I would guess that it calculates the MPG based on the fuel consumption over the last 100 miles (or conversely, the miles driven over the last 5 gallons consumed).

So, when you fill your gas tank up and calculate an MPG, it will be different than the reading on your dash because you're calculating the MPG based on an entire tank of gas (300 miles or so) and the car is basing it off of the last 100 miles. That is, unless your driving conditions are amazingly consistent.
I do not think you are correct. I have left trip odo "B" going for 8-10,000 miles at a time, and it is almost a glacial process to move it up or down one notch, whereas with trip odo "A" reset at the beginning of a new tank, it moves VERY quickly with driving behavior when there are very few miles on it, and gets more glacial when accumulated mileage is higher.

Whatever would make you think it averages over an arbitrary short distance like 100 miles? Do you have any supporting info on this? I just don't think that's the way it works.

George
 

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2010 FXT Limited 4EAT Sport Shift
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George
I would guess that he's speaking out of the same orifice as the rest of us :icon_razz: If the electronic fuel use device is using the odometer, as compared to the fuel guage for mileage determination, then following his assumption, the electronic readings would reflect the apparent inaccuracies of a 'faster' needle movement during the last half of the tank.
Unless I can be shown that engineers have solved the inaccuracies of fuel guages problem, I'll advocate for the empirical accuracy that comes from meticulously recording fuel consumption and miles travelled, over time, to get a smoothed (yes, engineers employ accumulative averaging) average fuel use.
Quick

I do not think you are correct. I have left trip odo "B" going for 8-10,000 miles at a time, and it is almost a glacial process to move it up or down one notch, whereas with trip odo "A" reset at the beginning of a new tank, it moves VERY quickly with driving behavior when there are very few miles on it, and gets more glacial when accumulated mileage is higher.

Whatever would make you think it averages over an arbitrary short distance like 100 miles? Do you have any supporting info on this? I just don't think that's the way it works.

George
 

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2017 Foz 2.5i CVT
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George
I would guess that he's speaking out of the same orifice as the rest of us :icon_razz: If the electronic fuel use device is using the odometer, as compared to the fuel guage for mileage determination, then following his assumption, the electronic readings would reflect the apparent inaccuracies of a 'faster' needle movement during the last half of the tank.
Unless I can be shown that engineers have solved the inaccuracies of fuel guages problem, I'll advocate for the empirical accuracy that comes from meticulously recording fuel consumption and miles travelled, over time, to get a smoothed (yes, engineers employ accumulative averaging) average fuel use.
Quick
I believe the fuel consumption gauge would use the odometer reading (or raw data provided by the speed sensor) but read the amount of fuel consumed via the injectors. There is a computer that tells them how much fuel to squirt, and that computer just total amounts squirted over time.

Differences will be due to fuel mass (due to temperature), both at the gas station and in the fuel lines. And certainly gas station pumps HAVE to be off by some amount. I need to talk to someone who has calibrated gasoline pumps to see how close they really read.

You get a reading within one mile of a reset of the system, and no way is the fuel gauge involved in measuring THAT.

George
 

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2010 FXT Limited 4EAT Sport Shift
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George-
There are small differences in fuel volume varying with the temperature of the undgerground tanks...but they are insignificantly small differences in the context of the accuracy of the devices we're talking about. There are of course isolated incidents where scoflaw service station operators will for a brief period of time mislead the public on volume pumped...but again, this is a small, intermittent, spatially inconsisten population.
In the face of our wildly varying driving behaviors, especially those of us with XT's with no boost guage, fuel mileage accuracy is kind of a futile pursuit, even with the increasing cost of fuel...if we really want better mileage, we'll trade off our Subies, huh?
-Quick

I believe the fuel consumption gauge would use the odometer reading (or raw data provided by the speed sensor) but read the amount of fuel consumed via the injectors. There is a computer that tells them how much fuel to squirt, and that computer just total amounts squirted over time.

Differences will be due to fuel mass (due to temperature), both at the gas station and in the fuel lines. And certainly gas station pumps HAVE to be off by some amount. I need to talk to someone who has calibrated gasoline pumps to see how close they really read.

You get a reading within one mile of a reset of the system, and no way is the fuel gauge involved in measuring THAT.

George
 
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