Calulated MPH vs Consumption Screen

The search function is your friend and giant font is not. lol

 

I hope you can see this normal sized font. If not, a magnifying glass may help.

Your question has been answered many times on this forum, so I won't go into a lot of detail. The short answer is that the volume of the fuel tank is inconsistent, so there is no way to do manual mileage calculations with any accuracy unless done over a large number of tanks. The MFD mileage is very good, typically within two mpg of long term manual calculations. The MFD always reads on the optimistic side, but this may be as much from pessimistic fuel pumps as optimistic MFDs.

Tom

 

If you use the auto-stop at a gas station, it will differ from station to station--or even pump to pump. I had this problem on my first test fillup (just a couple of gallons).I concluded the dealer on delivery had filled up more completely than my auto-stop. The consumption readout is much more accurate.

 

Shrug - first, as F8L implied: "giant font is not (our friend)."

Next - this topic is well hashed over in other areas. But short and simple, there will always be a difference because you will know the actual gallons you filled up with. Also, depending on weather conditions, especially in areas with varying climate patterns like summer vs. winter, it also affects the way the tank holds gasoline.
Over the life I had my 2005 Prius, I shows approximately a 5mpg difference... I got 5mpg more than the culculated. In my 2008 Prius, I have a 5mpg deficit - I am achieving 5mpg less than what its showing.
I can only suggest that everytime you fill up, you calculate this for yourself... save the receipt, record the mpg on the screen, gallons u filled up with (on receipt), miles drive (odometer/trip meter), etc. Keep in mind, that autostop thing on the gas pumps vary from station to station... so getting that "11.9 gallons" can vary too. This is where the error probably makes the biggest difference because you are calculating against what you are adding, not what you actually consumed. What you are refilling is not neccessarily equal to what you consumed like refilling a 1 gallon milk jug, the top ain't the same every time.
So far, on average, I have a +/- 5 mpg each time I refill. Sometimes more, sometimes less, but the average of the totals is about that. That's my experience - i think everyone gets a little something different depending on driving conditions. It will rarely, if ever, be exactly the same man.
Here's a summary of my old car's numbers:

Mileage: 27,635 miles
Gasoline Consumed: 520.5 gallons
Onboard computed MPG: 48.6mpg (avg of an avg)
Actual MPG based on totals: (27635/520.5) = 53.09 MPG

You can see the difference over time here but you don't see my actual spreadsheet. The weekly/monthly fill-ups have a wide difference, but you get that with almost any fill-up. Even your own daily driving will have a difference every day. If you track it everytime you fill-up, you will see that you yourself have a difference everytime you fill-up. I sometimes get 50 to 60 mpg and other times 40-45mpg. I used to drive a Nissan Maxima so there's still a little lead left in my foot when I showoff my Prius against something like a BMW 325 or Corvette...

 

Thanks for adding the rest fellas. I meant to add a link but I'm adding flora survey info into Excel and my boss walked by so I hit submit QUICKLY. lol