No improvement from Gen 2

Just think of it like a tire pressure gauge that always reads a bit high.

 

The dash reading isn't adjustable - I suspect they like it that way as it looks good; and it's never going to be accurate - but it's far better than early attempts. It can't be exact, as it's taking a lot of readings of fuel flow, distance etc and computing an average reading. And we know that you can't average an average and get a reliable result.

And it also varies according to a number of factors. Worn tyres can be up to 1½% difference I read somewhere, tyre pressures can make a difference to odometer readings.

 

You're being too kind and trusting, lol. At least on our 3rd gen: the bias is always in favour of "the house", on average 7.3 percent (currently, with over 160 calc'd tanks). I think similarly to the speedo (which reads high for some commendable reasons), the mpg display is intentionally optimistic (for not so commendable reasons). I strongly suspect the "internal" calc of mpg is close to spot-on.

More damning: the calculated vs displayed stays monotonously close to that 7.3% positive bias, tank after tank. Which says to me filling vagaries, phase of the moon, or whatever, is not much of a factor.

Our previous Honda Civic Hybrid had liter/100 kms, and it was either spot-on, or out by 0.1, and that was underestimating the calculated fuel economy.

Another "data point": Wayne Gerdes (cleanmpg) drove a pre-production 2016 model year Prius, and he noted the displayed mpg was spot-on. The car was scheduled to be crushed, about a week after he drove it.

A screen grab of the last year or so. I guess that error does dance around some, but yeah, invariably in favour of the house. There's a few outliers, fill-up vagaries I think:



Try to ignore those last few tanks, I'm REALLY struggling, due to new Michelin Primacy 215/45R17 MXM4's I think. I do think it's a matter of new rubber, squirmy tread: our current tank is starting to improve.

 

My spredsheet doesn't have the error column: I should add that.

 

Yeah I just read the displayed fuel economy off the dash and enter it, and up at the top I have lifetime kms and gas pumped, and the average error. The reason I've got so many fill-ups you'll notice: most of my tanks are around 25 liters fill up. I want an unbroken record, and the safe way to do that is ensure I'm the one always tanking up, saving the receipt, noting the odo reading and so on. So any time it's below half I fill up as soon as it's convenient. Which is still only about once a month, lol.

I reset trip meter at each fill-up, but that's mainly for feedback, to show the displayed fuel economy per tank. For the calc I enter the odo reading each time, have the spreadsheet subract last from current, and the diff is the kms per tank.

 

I use trip-A for each tank, and trip-B for various trips I want to capture.
I take pics of total mileage and trip-A before each time I get fuel, reset trip-A, and write the total mileage on each receipt.
I've never nerded-out so much on fuel data before.

 

61 fillups and only once has the car's computer understated MPGs. Pretty consistent in the error column.

 

I might be OCD - but not THAT OCD.


I temporarily noted when I filled last night "3.9" on TripA when I reset it - and the spreadsheet was 4.0 - then forgot it. But - hey, that doesn't make sense - if I forgot it, how come I still remember it. Maybe my short-term memory isn't that bad. Seriously, I always did have a good memory for trivial facts.

I did write it down for my first ½ doz fills. It was about the same discrepancy (~5%) as the FORDs and the KIA had.