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Fuel MPG - onboard vs old school

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been getting great mileage per the onboard computer. so i decided to calculate mileage the old school way. guess what. 30.0 mpg vs 28.4! what gives? for old school. i filled up. drove around until next fill up. filled up and did the mileage / gallons calc. so anyone want to join me ina class action lawsuit?
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The on board calculator is usually off by a little. It can be calibrated to read closer to actual, though it won't ever be perfect. You might want to start there before storming the castle.
 

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2021 MGM Outback 2.5i Premium with Tungsten Grey seats
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The variation on mine is dependent on the type of driving. If the tank is nearly 100% highway it's much closer to actual than during my normal commute.
 

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2012 Legacy 2.5GT-L
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anyone want to join me ina class action lawsuit?
Good luck with that. The odometer is about the only thing which has to be accurate.

Also - don't go with a single tank of gas - calculate for a couple and see where the computer says the mileage is and your calculated mileage is. You should be able to access the mileage setting via the following procedure:
Go fast...ignition on
turn headlights on
hit trip meter 5 times
headlights off
hit trip meter 5 times
headlights on
hit trip meter 5 times

Now you can adjust the MPG reading on the first option screen. Default is zero.
 

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2012 Outback 2.5i
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My AVG MPGs according to the computer are 1 - 1.5 high almost every time. I check my MPGs every fill-up and keep record. I'm avg. Just under 28 since the car was new. I never reset the "B" trip and thats about what it says.

On a slightly different note. There is a difference between my GPS MPH and the MPH guage, about 3-4 MPH.
 

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2007 Outback XT Ltd
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The odometer is about the only thing which has to be accurate.
Well, the speedometer and the odometer get the same road speed signal, and that depends entirely on the tire diameter. With new OEM tires your speedo and odometer _ought to be_ accurate, but they rarely are. That is why the gps reading shows a different value.

If your speedometer is high by 2 or 3%, so is your odometer. One reason I hate running undersized tires is that the odometer shows a lot of extra miles that never happened.

If you use your gps to measure miles traveled and your tank fill for gallons used, it can be significantly different from computer's calculated mpgs since the odometer may not be accurate...

An Ultra Gauge can be calibrated using a gps or road mile markers to give nearly 100% accurate trip readings.

John Davies
Spokane WA USA
 

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'13 Outback 2.5i CVT Prem. & '07 Tundra 5.7 V8 Ltd.
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All this being said, I can achieve the gas mileage promissed by toyota on my other vehicle. A little dissapointing if I can't with this one ... I will then hope that someone's nose is growing ...
 

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2011 Outback 2.5i Premium, CVT, Steel Silver, all-weather package. Upgrades: Tweeter kit, BlueConnect, media hub, remote start, Curt 2" receiver hitch.
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You can calibrate the computer. 30% of 30 mpg is 3 mpg and that is within spec. i saw the procedure, maybe on here? or googled something and i tripped over it.
Ummm... 30% of 30 is 9.

Yes, you can easily calibrate the inboard computer. I think I have mine set to -3 (that's the setting... It's not -3 MPG) and it seems to be very close now.
 

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2021 MGM Outback 2.5i Premium with Tungsten Grey seats
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All this being said, I can achieve the gas mileage promissed by toyota on my other vehicle. A little dissapointing if I can't with this one ... I will then hope that someone's nose is growing ...
The number on the sticker isn't a promise. That number is the EPA saying "we tested an example of this vehicle under certain conditions, here's what we got".

With that said, I HAVE achieved better, in less than optimal conditions, but I also don't trust the computer for the official number. I got 30.7 (computer read 30.9), the first 2 hours of that trip were spent stuck in traffic, the last 3 or 4 were spent doing 75 when I could. I believe my window sticker (I have a 2011) reports 29 for highway MPG.
 

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I've noticed the same thing. I'll have the cruise locked at 70 and my portable gps says 67-68. Also noticed when you pass the speed readings in construction zones always read 2-3 mph less than my speedo
Liability issue on the auto manufacturers part; every vehicle indicates a slightly higher speed than actual speed. Just imagine if the speedo was set to read "exactly" accurate, or worse, slower than actual speed. If you were pulled over for speeding or caused an accident because you were going faster than the speedo indicated, lawsuit. The couple MPH buffer zone (usually a %, not a hard-set 2 or 3 MPH) is to protect the manufacturer (and driver, sorta).

First post and a thermo-nuclear lawsuit?? Back under the bridge please.
+1, troll...
 

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2011 Outback 2.5i Premium, CVT, Steel Silver, all-weather package. Upgrades: Tweeter kit, BlueConnect, media hub, remote start, Curt 2" receiver hitch.
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As for the odometer and speedometer being in sync, this is not always true. Most German cars display about 2.5 MPH lower than actual on the speedo, but the odometer is correct. The ECU knows the true speed, but modifies that information when sending it to the speedo. If you have the proper diagnostic tool, you can view the actual AND "displayed" speeds side by side, and they are slightly different.

I don't know if Subaru does this, but it is very common with German cars.
 

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2011 Outback Limited 2.5i (sold 257,000km)/2018 Crosstrek limited 2022 Forester Limited
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...I use fuelly and do it old school too and find a difference from the onboard OBD computer. It's a little optimistic
 

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2019 2.5 Ltd.
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I would never trust an onboard computer mpg calculator. Why. Because I dont know how it is programmed and installed to work. I didnt think it would be accurate from the get go.
I do manual readings all the time and yes usually the computer display is too "optimistic" shall we say. Class action suit? Ridiculous.. Or a joke?
 
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