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Discussion Starter · #1 ·
As per the title does anyone know the technical reason why the computer mpg is so much higher than the actual mpg.

On my two fill-ups so far on both occasions the computer over estimated the mpg by 10-15%.

On previous cars the error has only been 5%. Why is the Civic so much worse?

I imagine fuel is regulated and monitored closely by the injectors, so struggle to imagine more than 1-2% error here.

I've not checked the speedometer accuracy. How accurate is it usually in the Civic? 5% I'd hope.

OK, even the full tank to full tank method for calculating the fuel efficiency has it's own error. But nonetheless up to 15% seems high to me.

Any thoughts?
 

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Premium Member
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Discussion Starter · #3 ·
OK, but why and how?

Fuel economy is only miles per gallon after all. Miles measured by the speedometer and gallons by the injectors. I as yet do not see the reason for the error to be any higher than 5%, not 15% in my case.
 

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It's all down to component tolerance and calibration. How in depth do you want to go? Your speedo will be approximately 10% over and fuel volume will be measured by a flow meter, which has it's own tolerance, and then the tolerance of the sensor input to the ECU, so when you get into the ins and outs depending on the components used by Honda, 15% I wouldn't say is that unusual.
 

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I wonder how accurate any of these are. The one on my Civic is about 10% (ish) out. The one on my Diesel Golf was much worse showing 54 on a real 47 mpg.

I tend to think an occasional "brim to brim" calibration is the only fool proof way but once you have worked out an average deviation for the computer, it does give you a rough guide and they are useful for comparing mpg over different journeys. I like the double trip meter for this.
 

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Helpee
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Pretty sad, but I've kept a record of my Civic MPG since I bought it. Over 17,500 mile the Honda mpg over reads on average by 1.2mpg (~2.5%). Not too bad really.
 

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#MrGrumpy
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I would guess the MPG errors come from the fuel measurements, as whilst the speedo is inaccurate by 10% the mileage isn't! Could you imagine a 10% error in mileage on a car that's done 100k miles?!
I've no idea where the fuel consumption rate is being taken from but it can't be a float in the fuel tank; it's far too responsive to throttle inputs for that to be the case.
 

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Pretty sad, but I've kept a record of my Civic MPG since I bought it. Over 17,500 mile the Honda mpg over reads on average by 1.2mpg (~2.5%). Not too bad really.
That's pretty good. Makes you wonder where the deviation not just between real world and computer mpg but also between different cars of the same make and model comes from.

Just a thought but my mileage involves a lot of uphill and shortish stuff so plenty of work through the gears. Must be difficult for any system to account for revs going up and down.

Could those who get more accurrate readings do more motorway stuff at constant speeds?
 

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I would guess the MPG errors come from the fuel measurements, as whilst the speedo is inaccurate by 10% the mileage isn't! Could you imagine a 10% error in mileage on a car that's done 100k miles?!
I've no idea where the fuel consumption rate is being taken from but it can't be a float in the fuel tank; it's far too responsive to throttle inputs for that to be the case.

That's an interesting one. I was thinking about this one the other day and I wouldn't like to confirm it but I'm sure there was a slight mileage devation between two points with my Honda and Ford of around 2%. I'd go out and test it but I no longer have the Ford.
 

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There are no flow meters, and no measuring by floats either.

The injectors are told to open many times a firing cycle for certain durations. So, if you have a good guess at the volume of fuel that flowed for the duration that the injector was told to be open, then you can have a good guess at how much fuel went in.

Now, how much fuel went in? It's subject to to fuel pressure, viscosity (temperature), injector hole size accuracy and opening time accuracy, algorithm accuracy and so on.

So it's an estimate, using a pretty inaccurate set of assumptions. Sometimes it's quite close, and sometimes way off.
 

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Also you are seeing an average over a certain duration.
 

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I would guess the MPG errors come from the fuel measurements, as whilst the speedo is inaccurate by 10% the mileage isn't! Could you imagine a 10% error in mileage on a car that's done 100k miles?!
I've no idea where the fuel consumption rate is being taken from but it can't be a float in the fuel tank; it's far too responsive to throttle inputs for that to be the case.

They're measured completely differently. There is no discrepancy between the millage your car says it has and what it has actually done. Unless you've tampered with it, of course.
 

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So why does the computer always over estimate the mpg?? Mine is alwasy 3-4 mpg over.. Anyone's under estimate?
 
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