I have found in the FAQs I can download my data as a CSV, so I can make the calcs in Excel if needs be. It would be good to go back and average the last 5 or 6 fuel-ups on here to get the 2500 mile average. That would be useful statistics to pull out of the data.
If you know roughly how many miles per tank you get, and how many tanks you use per month, then you could find out you answer. Example, I average about 700 per tank, one tank per month, so if I select for the last 3 or 4 months, I know how many MPG's I've averaged for the last 2100-2800 miles "ish" ha!
I've always thought a "custom" selection is worth having, so you could see your MPG'S from say November to March in the winter, and compare to the warmer summer months etc.