What you can do if you really want to ignore that data without losing the data from a full tank, is just fill up immediately before and after (as soon as you pick up the car) and ignore that fill-up.
That way you just throw out the data you think is bad. You don't have to wait until the tank is empty to fill up if you aren't going to use the data anyways. Throwing out a whole tank won't affect your mileage much, but it will affect your cost of ownership data.
What was the average for this tank that you threw out anyways? Just out of curiosity.
^^ Not sure what the average was. I had dropped the car off with an OBC (on-board computer) MPG average (pretty close to what Fuelly indicates) of 22.8 - 23.8 MPG. Got the car back with an 11.2 MPG rating. I can find the odometer info I think in fuelly somehow and manually calc out the average.
I would still add the fill-up otherwise it's kind of like cheating your way through the best MPG. You shouldn't skip any fill ups, I almost did that once. I get 10-20% worst mileage in the winter and I tend to idle my car during cold mornings when I have to go to work.
2013 Mazda 3 GS-SKY 6MT (Current)
2015 Mazda 3 Sport GX 6MT (Lease return)
2013 Mazda 3 Sport SKY-SKY 6AT (Ex's daily driver - totalled)
2007 Toyota Matrix Base 5MT (Sold)
I idle my car a bit before I leave for work too (I am in PA), but I count that. I just don't count the time the car is being driven in a manner other than usual. I think it makes sense. Like a manufacturer would only record MPG on a test track, not on the way to the test track.
My only point to add to this, is when you throw out that tank, you also throw out the amount you spent, so it affects more than your mpg. As I said before, the impact will be non-noticeable after you have had a few. I have had to get 3 tanks in a row at 12 mpg to actually cause the average to go down from 20.9 to 20.8. Crazy.
Its your fuelly, but still, something to think about.