Fuelly apparently doesn't believe a car can get 800 MPG, so it's not listing it as an accurate fuel up. But a PHEV can easily travel several thousand miles without fueling up.
I filled my car Monday morning before I went on a long road trip. Since I had be driving almost exclusively in EV mode, I only needed 2.45 gallons of gas to fill up after 1669 miles, for a calculated 678 MPG. But both the app and the web page doesn't reflect that fill up in the average MPG or the highest MPG. The app does show the MPG in the log, but the web site does not.
Fuelly should be accepting the calculated MPG, regardless if it thinks something is amiss. It does ask if I had perhaps missed logging a fill up, but when I tell it that is not the case it should just accept the numbers as entered.
Here's my web page for this car:
https://www.fuelly.com/car/kia/niro/...c98092/1287516