Fuelly calculations not matching car's

[pauhana658]

I have a 2008 Prius, bought second-hand here in Japan a few years ago. I get pretty good mileage (kilometerage?) according to the car's calculations (between 22-25 km/l) but when I enter the car and gas-pump figures into Fuelly, Fuelly reports significantly lower rates, between 1 and 2 km/L lower. I can do the calculations by hand and the figures match what I expect, so I'm wondering how Fuelly calculates its mileage. I was told by the previous owner that the car has bigger tires (19"?) than the standard Prius (__?) and that those bigger tires actually lower the mileage. Fuelly doesn't seem to account for tire size, so this discrepancy in Fuelly is even more puzzling. Any ideas?