mpg?

something is definitely off, but I dont think it's your car

(hint: 398 gallons of gas)

 

Nothing is off. There's a bladder in the gas tank.

 

Nope, nothing is off. Sounds about right.

I assume you mean 48.7 mpg on 398 miles, that the gas gauge was down to 1 bar (pip), and that you managed to get 9.49 gallons into the tank when you filled it up.

One pip on the gas gauge sounds like a good time to fill up. Adding 9.49 gallons when the gauge shows one pip is not unreasonable. 48.7 mpg seems resonable depending on your personal driving habits, topography, prevailing weather, etc. And 398 miles on one tank when you are getting 48.7 mpg isn't surprising either.

 

That's a huge tank! Must be the European model.

 

I think the OP meant
398/9.49 = 41+mpg while the computer shows 48+mpg. Something is off.
I am guessing that you are using the fill-up method to compare the computer MPG.
Two factors here:
- the pump click-off points are different from station to station, even pump to pump
- the bladder can change its volume depending on temperature
Combining both, you could see 10% error. However, I can't explain the big difference
you are seeing. Maybe .... don't go to that station again. Their pumps seem to be off the chart. Usually when the final pip blinks, I can only put in around ~9 (US)gallons.

 

Trust the MFD. The amount of gas you put in at the last fillup is almost never the same amount as you have actually burned since the previous fillup, so the manual method will always be inaccurate for a single tank. The variable-capacity tank bladder in the Prius makes this effect much larger than for non-bladder cars.