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Discussion Starter · #1 ·
I have a 1.5 DCT XCeed and I have to say that I am totally happy with it. Downsizing from a Sportage I did have some concerns, but all is well.

However, one little thing is driving me bonkers, the onboard Trip Computer.

I have covered just over 4,000 miles since taking delivery of it and the onboard Trip Computer states that my average fuel consumption has been 50.8 mpg, whereas my distance & quantity of petrol calculations reveal that the true consumption is 46.8 mpg, a figure that I’m totally happy with.

Fortunately it’s a very simple calculation, miles covered divided by number of gallons (or litres if you prefer) put into the car. Over a single refill it might be slightly out due to a variety of factors, but these should all be smoothed out over the larger distance.

I certainly don’t feel the need to take the car to the dealership to get it investigated but a simple calculation by the car’s on board computer should not be inaccurate to the tune of almost 10%.

Anybody else notice a similar inaccuracy?


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I use Fuelly to log my fill ups.


Car is showing 54.1 at the moment, reset to zero when picking car up. So pretty good as Fuelly is only up to last fill up & MPG on this tank is not as good so far due to colder weather & shorter runs.
Just over 5,000 miles done so far.
 

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With my Optima, I always reset the average mpg display each time I refuel. The more accurate brim-to-brim calculation is invariably a little better than the display figure but has varied during the last 5 years between (in round numbers) 37 and 48 mpg. The combination figure over the piece is a little over 43mpg which seems reasonable to me, given the space within the SW and the pretty comprehensive array of gadgets and gizmos KIA chucked in as standard.
 

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The onboard computer is never totally accurate, how can it be, too many variables such as tyre wear.

Worst we have had was a BMW that displayed about 53 mpg when in truth the car was doing about 47 mpg (which is what we expected.

Our Ceed displayed about 50 mpg but in truth it was nearer 52 mpg, the only car we have owned that under read.

Treat it as a toy and only believe your calculations. I use a spreadsheet, saves the calculator batteries.
 

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The onboard computer is never totally accurate, how can it be, too many variables such as tyre wear.
I agree except the amount of fuel used is probably the biggest variable, we measure brim to brim which might easily vary by 3 litres or more but the 'computer' is measuring the amount or fuel used, tyre wear shouldn't really effect it as we use the same recorded miles as the computer does, or at least it should be 🤣

Surprisingly for the nerd that I am I have never checked the fuel computer against a brim to brim calculation on any other car AND I have only checked one tank full so far which cannot be representative but I am disappointed to find that on this one calculation the car is more than 12% out, particularly as the average has been over 40mpg which I find spectacular as this is almost exclusively town driving, now I find it is probably only 33 mpg :mad:😭🤣
 

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Discussion Starter · #7 ·
I am disappointed to find that on this one calculation the car is more than 12% out, particularly as the average has been over 40mpg which I find spectacular as this is almost exclusively town driving, now I find it is probably only 33 mpg :mad:
Don’t be too disappointed, my real life average is about 48mpg, and I find I can exceed 40 without even trying. IMO that’s spectacular consumption for what is basically quite a lively car.

My post is in no way intended to be a criticism of the actual consumption, more a case of highlighting something going awry in the calculation.


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Remember the computer isn't just using miles travelled but idling, richer fuel under cold conditions and so on., But then if it's exaggerating then yeah, sneaky car.
 
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Discussion Starter · #9 ·
Remember the computer isn't just using miles travelled but idling, richer fuel under cold conditions and so on., But then if it's exaggerating then yeah, sneaky car.
In theory my calculated consumption and that reported by the car should be more or less the same, as they will both include periods of idling, reversing, starting up etc. 5,000 miles in and they are discernibly different to the tune of approx 10%, which isn’t a deal breaker but it is a bit naughty


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Remember the computer isn't just using miles travelled but idling, richer fuel under cold conditions and so on., But then if it's exaggerating then yeah, sneaky car.
Except on over run it is using no fuel.. Other times the mixture will be leaner.
 

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My spreadsheet over 135 fills (invariably to the top) shows:

On board average: 39.3mpg
Calculated average: 38.0mpg

So, about 3.5% over. Yet the speedo is 10% out, so go figure...
 

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Discussion Starter · #13 ·
My spreadsheet shows over 135 fills (invariably to the top) shows:

On board average: 39.3mpg
Calculated average: 38.0mpg

So, about 3.5% over. Yet the speedo is 10% out, so go figure...
Something somewhere isn’t being measured very accurately, which in itself isn’t the end of the world, but it does make the three separate trip/consumption displays pointless and redundant


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The speedo is wrong but the milometer tends to be accurate.
The speedo will be wrong. They are deliberately designed to read about 2-3mph fast at a true 30mph (or maybe 50kph!). This is done to reduce the risk of a claim of "I was misled by the speedo officer..."i. I think you will find that all manufacturers build them to read fast.
 

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The speedo will be wrong. They are deliberately designed to read about 2-3mph fast at a true 30mph (or maybe 50kph!). This is done to reduce the risk of a claim of "I was misled by the speedo officer..."i. I think you will find that all manufacturers build them to read fast.
All my recent cars overead by about 10% which is quite handy for an easy mental calculation to set cruise to be 'just legal'... I always assumed that mileage was similarly affected but I suppose now that I actually think about it, that would make no sense whatsoever!
 
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That is good mpg. I use Fuelio app for brim to brim. Per the app for last 3300 miles, the mpg has been 46.11mpg and the car display shows 47.3mpg.
My old Golf display used to show 7% more than the actual. So Kia's seem to be a bit more accurate.
Also when I fill petrol, i fill it up consistently ie i always stop at the first time fuel dispensing auto cuts off. Of course, you can still a bit more fuel after the the first cut off.
 

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My old Golf display used to show 7% more than the actual. So Kia's seem to be a bit more accurate.
Well, VW do have a history of being economical with the truth.
 

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I can't say precisely for a KIA as I haven't driven enough of them to compare, but taking experience from many miles driving trucks-
The declared mpg in different trucks of the same model can and does vary in accuracy, sometime by a quite large percentage, (that's accuracy of how much diesel has to be put in the tank not the variability of mpg due to load, speed, road type, weather etc)
The mileage and speed are measured by how many times the wheel goes round on the journey. This means that the precise measurement relies on the circumference of the wheel and as the tyres wear out the circumference changes by about 3%, that makes both the mileometer and the speedo 'inaccurate'. For the purpose of comparing onboard and spreadsheet mpg that inaccuracy is irrelevant as the 'mileage' used to calculate both will be the mileage that is declared on the dials. Experience in many vehicles would tend to show that the speedo and mileometer usually read faster/further than reality when timed against known distances or against satnav speeds but that is not always the case, so don't assume that you can have 33 on the speedo in a 30 limit.
Once one has allowed for the inaccuracy of measuring the distance, for speed the other unit of measurement is time, on a modern electonic speedo the time element will be from a digital clock which is generally accurate to within a few seconds a year so not really relevant.
To calculate mpg there need to be some measurement of fuel used and I expect that is done from the cars computer controlling the fuel injection system and a calculation of how much fuel is squirted into each cylinder for each stroke, that has to be done thousands of times each minute in real time and will depend on a flow rate calculated from fuel pressure and injector size multiplied by the small time each injector is open, with those all being very small measurements the inaccuracy of the measurements will come into play much more. When you buy fuel at the garage if you buy 50 litres measured to 0.01 litres then you have an inaccuracy of 0.02%, if you use a spreadsheet for your calculations over 10 refills then it goes down to 0.002%, compare that to the fuel injection measurement/calculation where each shot of the injector is hundredths of a millilitre (less than a single drip from a tap) achieving 0.002% accuracy would require measuring/calculating to millionths of a millilitre, and remember that is done thousands of times a minute in real time as you drive with the mpg then being continually recalculated, it's pretty miraculous that we get an mpg reading at all even if it is a bit inaccurate.

I tend not to be too fussed by the mpg figures of my cars, after all I've bought the car so I'm stuck with it and its mpg, variations in actual mpg are down to me and my heavy right foot, the one figure that I do use is the 'miles to empty' prediction but then I only use it as a rough guide and want to be filling up or know where the nearest petrol station is when it gets down to 50 miles.
 
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