what's my MPG?

Because of the bladder fuel tank, the Prius does not reliably fill to the same level each time, making it impossible to calculate an accurate mpg reading on each tank. To obtain the actual mileage, divide the number of miles recorded on the odometer by the number of gallons put into the tank for several tanks. Averaged over a few tankfuls the number will be your actual gas mileage for that number of tanks. On a tank by tank basis, use your MFD displayed mpg and reset at the start of each tank or tanks as you prefer. The MFD displayed number is quite accurate as the computer monitors the fuel flow and divides it by the miles driven. The odometer is very accurate as long as tire diameter hasn't been changed.

 

The short answer to your last question is yes.
Fuel gauge is pretty accurate, but it varies a bit based on temperature and sunspots or something, which throws off the people who want to compute it to one or two decimal points. You can average several tanks like direstraits says, should be within a percent or two of the MFD.

 

Yeah, there's not much of a way to tell using that method. I went about 370 miles on my last tank and filled about 5.8 gallons before I clicked. If that were truly accurate, it would mean I was getting about 64mpg or so. My consumption screen says I got 51.something mpg, if I remember correctly. From what I've read, the consumption screen is pretty accurate, so there's evidence right there that calculating based on how much gas you fill from tank to tank tells you nothing.

 

the mfd is the way to go. You can be dead empty on the gauge and take 10 gal or you can take 8 gal. the tank bladder is crazy.

believe the mfd. it is your friend!