No MPG on next tank after partial fuel-up ?
 

After partial fill up, next tank does not show MPG. Why not compute MPG on the combine tanks by totaling fuel and mileage ?

Because Fuelly calculates an individual tank by measuring the difference from full. Without a full tank to work with, there's no way to know the difference. You could put in 2 gallons and then 3 gallons and then 4 gallons continually, but Fuelly can't calculate an individual MPG because there's no difference from full.

We do use those fuel-ups toward calculating the overall average, though, so it's still worth adding them. There's a bit more info here and here.