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2014 Forester XT Premium CVT
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81 Posts
Discussion Starter · #1 · (Edited)
I've been closely tracking my gas mileage using both fuelly and with the trip computer for my first 4 tanks of gas on my FXT. So far, fuelly is showing the following:

21.4
23.5
23.0
Overall: 22.7 MPG

My trip computer showed 25.2 on my last tank. That seems pretty far off. The previous tank showed 25.5. Overall, it's showing 23.5 So to recap:

Fuelly TC
23.5 25.5
23.0 25.2
22.7 23.5 (overall)

Anybody else's trip computer way off? I've owned three cars in the past that showed the fuel economy and all were far more accurate. A VW, an Audi and a Jeep. All were within .5 mpg. Even my 2000 GTI, you'd think in 14 years they could have made some improvements....

 

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Premium Member
2014 Forester XT Touring CVT
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12,780 Posts
Yep.

Mine is usually 1-2 MPG off.

I think the computer struggles to detect high fuel usage under short periods of boost (from the turbo) so it tends to be more optimistic than it should be.

My last tank showed 30.6 but actual MPG was 27.9.

Long story short, it is best used as a guide to gauge your consumption trend rather than a precise fuel economy gauge.

I am also tracking mine on Fuelly.

 

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2012 Forester X 5 MT
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881 Posts
I notice that MPG displays are always optimistic, never pessimistic. Maybe the marketing boys have a hand in this.

Mine is a bit less than 1 MPG optimistic. This has to be averaged over several tank fulls to have any real meaning.
 

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2011 Forester
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107 Posts
Mine says I'm getting 19 in town and 23 on the highway. I calculated it out and it is getting 14 in town and 18 on the highway.


Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.
Benjamin Franklin
 

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2016 Forester 2.5 CVT
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132 Posts
Why even worry about it? Drive your car and enjoy it.
 

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2019 Tacoma TRD OR 4x4 6AT
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541 Posts
I remember seeing my "miles before empty" reading at 670 miles the other week, so I would tend to agree that the MPG calculations seem questionable.
 

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2018 Forester 2.5iLimited CVT
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242 Posts
Of course the "normal" way of computing MPG is to record mileage and fill up amount and doing the calculation. But, why do you believe there are not more errors in the standard way than the computer? Most people gauge their computer accuracy by the "normal" method and say their computer MPG is optimistic, but which method is actually most accurate? If you do many tank fills over a large number of miles, then maybe the "normal" method is more accurate, but just how accurate are the gasoline pumps? When you are talking 20+ MPG, a 5% error leads to +-1+ MPG even by the normal method of calculating.
 

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2014 Forester 2.0XT CVT
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181 Posts
since it is off always to the plus side you can bet Subaru has a play in it, I bet most other suv forums have same discussion about their cars accuracy, has always been marketing high mileage to sell the cars, even cheating on the sticker averages.

Honda for one had to pay millions in rebates for false claims
 

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2010 Forester Active 2.0X MT 2x5 - MT with Dual Range reduction
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1,236 Posts
I notice that MPG displays are always optimistic, never pessimistic. Maybe the marketing boys have a hand in this.

Mine is a bit less than 1 MPG optimistic. This has to be averaged over several tank fulls to have any real meaning.
For the most part I confirm that.

Real fuel consumption of my Forester 2010 mostly 0.2l/100km more than displayed by the trip computer, driving some days only in the city ca. 0.4l/100km more than displayed.

0.2l/100km = 0.46MPG
0.4l/100km = 0.91MPG
 

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2019 Crosstrek 2018 XT
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14,698 Posts
But, why do you believe there are not more errors in the standard way than the computer? But just how accurate are the gasoline pumps? When you are talking 20+ MPG, a 5% error leads to +-1+ MPG even by the normal method of calculating.
Gas pumps are calibrated with accepted standards. If you fill up say two tankfulls at the same pump, you will be much more accurate than the computer.

The computer actually monitors the gas delivery of the injectors. Not saying this is inaccurate, but it will note as accurate as the "normal method."
 

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2013 Forester X Automatic
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200 Posts
My 13 is always 0.7 mpg optimistic. I don't worry about it, it just allows me to know how it's trending. I think they are typically optimistic so that the manufacturers don't get a barrage of complaints. Imagine if they consistently read lower then actual mileage.


Sent from AutoGuide.com Free App
 

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2010 2.5X Limited 4-speed Auto
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Why even worry about it? Drive your car and enjoy it.
My philosophy, as well. I have never understood the fixation with tenths of a MPG; It is what it is. The only time I pay attention is if there is a significant change.
 

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2018 Forester 2.5iLimited CVT
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242 Posts
Gas pumps are calibrated with accepted standards. If you fill up say two tankfulls at the same pump, you will be much more accurate than the computer.

The computer actually monitors the gas delivery of the injectors. Not saying this is inaccurate, but it will note as accurate as the "normal method."
You could be correct, then again, you might not. But, blanket statements without any supporting evidence are not useful...just opinions.
 

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2014 Forester XT Premium CVT
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81 Posts
Discussion Starter · #17 ·
Of course the "normal" way of computing MPG is to record mileage and fill up amount and doing the calculation. But, why do you believe there are not more errors in the standard way than the computer? Most people gauge their computer accuracy by the "normal" method and say their computer MPG is optimistic, but which method is actually most accurate? If you do many tank fills over a large number of miles, then maybe the "normal" method is more accurate, but just how accurate are the gasoline pumps? When you are talking 20+ MPG, a 5% error leads to +-1+ MPG even by the normal method of calculating.
Gas pumps are tested regularly for their accuracy. I worked at a gas station all through high school, I have seen it done. Hell, even the station itself checked the pumps from time to time.

If the calculation method was inaccurate and I did every tank, it wouldn't make sense that every tank was a few MPG less than the trip computer. It should all even out in the end.
 

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2018 Forester 2.5iLimited CVT
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242 Posts
Gas pumps are tested regularly for their accuracy. I worked at a gas station all through high school, I have seen it done. Hell, even the station itself checked the pumps from time to time.

If the calculation method was inaccurate and I did every tank, it wouldn't make sense that every tank was a few MPG less than the trip computer. It should all even out in the end.
What were the accuracy requirements? Also, do we know that the shut off is accurate in its detection of a full tank? How accurate does that have to be, or are there any requirements for it? I know most states check pumps for accurate delivery and put seals on them, but does anyone know what the accuracy requirements are? But, even if a pump gives 1.00 gallons for 1.00 on the pump, when filling up most people use the pump shut off to stop pumping. Also, what fluctuations exist in the detection of a full tank by the shut off? All of these factors come into play when saying that the "normal" method of calculating MPG is more accurate than the car computer.
 

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2010 2.5X Limited 4-speed Auto
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To some no...to others yes...just like many other topics on the Forum.
It must have been really dull on forums such as this before the days of on-board computers when we all drove around in blissful ignorance. :woohoo:
 
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