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Discussion Starter · #1 ·
I just clicked over 2,000 miles on my new '17 WRX.

Each time I have calculated MPG from the trip odometer and actual fuel fill, the MPG has come out about 10% poorer than the dash display. For example, yesterday the odo was 361.5 miles, gas purchase to fill was 14.494, so MPG 24.94. Dashboard computer claimed 27.3. so the real world was about 9% lower than the claim.

Speedo, checked against GPS, is the most accurate I have ever seen. Clearly within 1%, so it is almost certainly not an odometer issue. Apparently either the fuel flow sensor is badly calibrated or the software is deliberately lying to me. A 10% error in fuel flow measurement is really high; my experience is with light airplanes where errors are maybe in the 1% range. I don't think the airplane flow meters, almost certainly turbine meters, are going to be much higher cost/quality than what is in the car, as these are all low-end meters.

Is anyone else seeing this?
 

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i thought it was just generally assumed that the MPG counters in cars read a little off, usually within 10% variance? Or at least that was the case for our cars, from what i've seen and read.

in for any more educated answers though
 

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Discussion Starter · #6 ·
...seems like it could be the float device they use to measure your fuel level, depending on temps it could be off by as much 4%
Thanks for the link, but most of that article is nonsense.

" ... inaccuracy is unavoidable. 'The density of fuel varies. We have no way to measure it.' We are talking volumetric flow here, not mass flow. Density is irrelevant.

“We look at the fuel-consumed data that comes from the engine-control computer, but we also track the float sensor measuring the fuel level in the tank, ... ” Tank level measurement using floats is hopelessly inaccurate as a means to measure fuel usage. One obvious reason: As the tank is filled, the float ends up at its top limit and totally immersed in fuel. Only when the fuel has been drawn down to the point where the float is actually floating does the float level mean anything at all. That is why the gauge stays on "F" for a long time before it starts to move. Honda is probably "tracking" the float level only to trigger the low-fuel annunciator light.

One interesting conclusion from the article, though, is that it is not just my Soob that is lying. Apparently most cars are lying. And the fact the error is always to overestimate MPG leads easily to the conclusion that it is intentional.
 

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Thanks for the link, but most of that article is nonsense.

" ... inaccuracy is unavoidable. 'The density of fuel varies. We have no way to measure it.' We are talking volumetric flow here, not mass flow. Density is irrelevant.

“We look at the fuel-consumed data that comes from the engine-control computer, but we also track the float sensor measuring the fuel level in the tank, ... ” Tank level measurement using floats is hopelessly inaccurate as a means to measure fuel usage. One obvious reason: As the tank is filled, the float ends up at its top limit and totally immersed in fuel. Only when the fuel has been drawn down to the point where the float is actually floating does the float level mean anything at all. That is why the gauge stays on "F" for a long time before it starts to move. Honda is probably "tracking" the float level only to trigger the low-fuel annunciator light.

One interesting conclusion from the article, though, is that it is not just my Soob that is lying. Apparently most cars are lying. And the fact the error is always to overestimate MPG leads easily to the conclusion that it is intentional.
Not likely intentional. More likely the result of averages. I can roast through a tank of fuel super fast with hard acceleration then coax the meter to read high with more miles of highway driving.

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Discussion Starter · #8 ·
Not likely intentional. More likely the result of averages. I can roast through a tank of fuel super fast with hard acceleration then coax the meter to read high with more miles of highway driving. ...
That won't make any difference in the final reading unless the flow meter output is significantly non-linear with flow rate. i.e., under- or over- reporting depending on flow rate. If the measurements are accurate then the sum of the measurements will be accurate.
 

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That won't make any difference in the final reading unless the flow meter output is significantly non-linear with flow rate. i.e., under- or over- reporting depending on flow rate. If the measurements are accurate then the sum of the measurements will be accurate.
It does. I assure you, go test it yourself.

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I didn't come here to argue. There's no theory to test. It's simple arithmetic. If the total of the measurements varies depending on flow rates then the sensor has a nonlinearity problem. Believe what you like.
So, basically you have a simple method of testing a theory, refuse to, and just crap on information that has been collected. That's here to argue and not arrive at a conclusion.

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There have been many threads about fuel mpg on here. For the 15+ cars, OP, it is exactly as you say (in general) that they get ≈ 10% less mpg than indicated. I check mine every fill-up and it is always the case. An algorithm (with various sensor input) is used to feed the display. My personal belief is that the algorithm is intentionally made to yield high. Why? Not sure. But I'd guess it is a competition thing with other cars and such readings are either not regulated or regulations are vague with large tolerances.
 

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Discussion Starter · #18 ·
There have been many threads about fuel mpg on here. For the 15+ cars, OP, it is exactly as you say (in general) that they get ≈ 10% less mpg than indicated. I check mine every fill-up and it is always the case.
Thank you. That is exactly the information I was looking for.

An algorithm (with various sensor input) is used to feed the display. My personal belief is that the algorithm is intentionally made to yield high. Why? Not sure. But I'd guess it is a competition thing with other cars ...
Of course. It's good advertising if the consumer believes the car is telling the truth and brags to others about the bogus numbers. The problem is the damned engineers that think they have to check everything they are told.

Somewhere in the dashboard there is a calibration coefficient. I'm going to talk to the dealer about whether they have access to it and can tweek it to achieve accuracy.
 

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Thank you. That is exactly the information I was looking for.

Of course. It's good advertising if the consumer believes the car is telling the truth and brags to others about the bogus numbers. The problem is the damned engineers that think they have to check everything they are told.

Somewhere in the dashboard there is a calibration coefficient. I'm going to talk to the dealer about whether they have access to it and can tweek it to achieve accuracy.
Keep us updated. What I've noticed with our 17 forester is it is pretty accurate on a single trip basis...notice every time you turn the car off it will state how many MPG you got for that trip. I've played around a little by filling up the tank, driving to the store, logging that #, driving back to the gas station, logging that #, taking the average and then comparing the 2 averages to the actual fuel consumption miles/gallons and it was pretty close; like .8mpg off.

Now think about doing this for an entire tank, there are probably 20+ start/stops. I believe the entire tank consumption tracker, rather then tracking mileage, uses a much more basic algorithm of the averages, and maybe even weights the means, of all the trips combined to give you an estimate.
 
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