Well as you're here, you might as well start tracking your mileage with fuelly. It will give a far better more accurate result than any other calculations, and you can see your true costs and averages for fuel consumption. Its fun too
My 2015 1.6L Stick Shift Fiesta is consistently averaging 50-55 mpg.....City or highway.
Are you pumping up your tires & using 100% ethanol-free gasoline(E0)? Do you have hills(even shallow ones) or know how to handle hills so your mpg doesn't fall much? Did someone bless you, so that the wind was always at your back?
My guess is that the OP either has a l.0l SFE (2014) or may be inflating FE estimates. Only the 1.0l are in the upper 40s or higher - at least posted on Fuelly.
My BS radar is going wild... OP has no fillups... no data... there's no way that a Fiesta is getting 56 US MPG (67 UK MPG) over an entire tank driving at 80mph. Even driving at 55mph constantly with no traffic or braking you would be lucky to break 40 or 45 at the most. The base model Fiesta is rated at only 37 mpg on the highway, and if you're going 80mph there's zero chance you're going to even meet the highway figure, much less beat it by over 50%.
Either the OP is mistaken or is pulling a Draigflaig and using different units from the rest of us and forgetting to mention that.