Calculated MPG consistently higher than reported on MFD

The car knows when you're sweating bullets and running on fumes.....it goes into a special gas saving mode to give you extra miles to get to the gas station. I fill up with 2 pips remaining so I don't get this special gas saving mode on my car.

My best guess, when you're on fumes, your car is lighter than a comparable Prius and you'll eventually get a few extra miles more

 

Perhaps topping off your tank affects your mileage? You might be losing fuel due to spillage and evaporation.

 

My Persona has been anywhere from 5% optimistic during the summer months all the way up to I believe 8-9% once in the dead of winter...

So yeah, being the opposite is good I guess. Lol. I can't explain why though. Maybe as stated above it had to do with the over filling and not registering or something...

 

perhaps 5 or 6 tanks, calculated after refilling would give you a better idea of real mpg's.

 

+1. It is likely the tank fill up issue due to incline. Try to point nose down the slope when fill up.

Alternatively 46MPG seems to be low, perhaps there is some issue with sensors?

 

As I recall Prius uses MAF readings to calculate MPG. The reason why normally calculated and actual MPG differ is due to pump calibration and transitions when the mixture differs from 1:14.7. If your calculated is much lower than real, perhaps issues with MAF sensor readout? I'd go to dealer and have it checked.

 

Is it still happening to you? Here are my last three fill-ups. Keep in mind that I travel around 6000 miles per year:

November 11, 2015: My car's trip monitor said 343.2 miles at 54.1 mpg.
Since my fill up that day was 5.665 gallons, that should be 60.6 mpg, so the computer in my car calculated a 10.7% lower mpg.

November 29, 2015: My car's trip monitor said 194.8 miles at 48.4 mpg.
Since my fill up that day was 4.764 gallons, that should be 40.9 mpg, so the computer in my car calculated a 18.3% higher mpg.

December 20, 2015: My car's trip monitor said 138.0 miles at 43.9 mpg.
Since my fill up that day was 3.444 gallons, that should be 40.1 mpg, so the computer in my car calculated a 9.5% higher mpg.

 

over 5 fillups I am averaging about 1.5% difference, my calculated mileage is slightly higher than the C's computer, very consistent

 

I've done 23 fill-ups so far and I'm averaging about 5% worse fuel economy measured by the amount of fuel pumped vs. the fuel economy displayed on the car's information screen. Not a big deal, as far as I'm concerned.

 

Yup sounds about right.

I averaged between 5-9% off (summer / winter) in my old persona.. For reference.

Haven't been tracking my wife's v's mpg but on our last 200 mi road trip we averaged like 34 mpg with 3 people and some crap in the trunk. Not the greatest but not the worst I guess at 70-75mph..

 

I'm facing exactly the same situation.. Well calculating based on L/100 km method (Australia), I am told by MFD my economy to be 5.9L/100km which is pretty bad for a Prius.. I was so disappointed until I decided to check it manually. When I did, it turned out to be 4.9L/100km which is pretty normal for Prius C. I also noted that when I fill up the tank I always ended up around 4 ltrs less than the claimed tank size. Then I decided to take the risk of not filling up even after fuel fill up light began to blink.. and went 120 km more trusting my instincts...! Finally when I filled up, it matched with the claimed tank size and fuel economy (manually calculated) came around 4.9L/100km despite MFD still showing 5.9L/100km.
I don't know whether this is a car specific issue (such as fuel injectors injecting less fuel thus saving more in the tank while car computer thinks it has run out of fuel) or common for all Cs.. But I'm glad to know someone else has also experiencing the same so I'm not alone on this one..

 

it's about time you got in line, welcome to the club.

 

If you're expecting the standard automobile to behave like a scientific calibrated instrument, I'm afraid you'll be sorely disappointed. Live with your expectations firmly ensconced within the known values and accuracy as your car has already provided, and you should be good to go. Expect better from something that you know it cannot provide and ... what's the point?

However, let me add this to the discussion. I have three measurements I use when I feel like figuring it up. 1) What the car tells me the mpg is/was, 2) what my ScanGauge tells me is going on, and 3) gas/mpg and a calculator. If I were to figure the tank at, say, 50mpg with the calculator and the gas station receipt, the Scangauge would tell me it was 49-ish and the car would tell me it was 51-ish. Every. Single. Time. It's just the way it is. So, I go with the calculator and then run the others through the interpretor algorithm. No worries, since I'm largely done with MPG contortions and heroic gyrations for the sake of it.

 

I've noticed this too. My display may read 47.1 for a fill up. When I entered in to the Fuelly app it calculated out as 49.6. I'm fine with this discrepancy.