Looking at your Fill-Up Log:
http://www.fuelly.com/driver/waditz/x1/log
On Sept 14 you had driven 1 mile, and filled your tank with 16 gallons of fuel.
On Sept 19 you had driven 343 miles and put in 16.6 gallons (but you didn't top it off, as you marked it as a Partial fill).
On Oct 21 you had driven 330 miles and filled the tank with 15.18 gallons but because the previous fill-up was a Partial-fill neither entry will calculate a MPG reading.
If you look at it like this (as Fuelly does) you can see how misleading/confusing the data actually is. Please make the necessary corrections to your entries.
I'd start with editing/deleting your first entry as it's not necessary. The Sept 19th fill should NOT be marked Partial if it was indeed filled-up by the Dealer. IF you're uncertain whether they actually filled it up completely, I'd still opt out of marking it as a Partial and drop it completely.
You can see by looking at the November and January fills how the math should be performed. I'm pretty sure you're calculating your MPG by dividing by the gallons used for the previous fill.
Simply put:
(Mileage B - Mileage A) / (Gallons) = MPG
Where B= odometer reading at second Fill-Up;
A= odometer reading at first fill and;
Gallons= used to completely fill 2nd fill-up.
In your first post, I think you were using the Gallons filled during fill-up A.
Hope this helps!
Linear Mode
