Dear Harvey Anderson,
Thank you for your post and for bringing the issue up.
We have done a great deal of research and believe we found an underlying reason for those miscalculations.
It looks like fill-up history for your "2013 Ford C-Max" actually contains fill-up records for 3 different cars - one starts on 08/11/2008, second starts on 09/09/2012, and third starts on 06/18/2013, judging by odometer readings jumping down on each of those dates.
Unfortunately our site designed to work only with history for one car at a time, so record from 09/19/2012 for mileage 5,799 confuses our system and it uses that mileage 5,799 as previous mileage for 08/26/2014 fill-up with mileage 5,864 and calculates miles driven as difference between those 2 fill-ups, resulting in 65 miles driven and MPG value way off.
We suggest you either remove older records leaving only those for current vehicle, or create 2 other vehicles and we can help you break up your fill-up history into history for 3 vehicles.
We can gladly do that for you if this idea sounds good to you and you wish us to proceed, so please let us know.