I'm a Fiesta Newbie, but I hope you don't mind me commenting on your method.
Running your tank down to empty and using the tank capacity as a basis isn't going to be the most accurate method. The most accurate method (on any car) is as follows:
1. Fill tank (brim). Zero trip meter or make note of mileage.
2. Drive until tank is nearly empty.
3. Fill tank again (brim). Make note of mileage travelled or trip meter readin. Make note of litres used to fill tank - this will be the actual fuel you've used over a known distance, because you are filling up to the same point you started with.
Divide litres filled by 4.54 to get gallons used; divide miles travelled by gallons used to get accurate mpg (for that tank). Rinse and repeat as the saying goes, for longer term averages.
I got 44mpg out of the first tank on our Mk6 1.2 Finesse - mix of motorway & urban. Reasonably happy with that.
Hope this helps.
I already know this.
Just seemed easy to get a resonably accurate figure with my method.
A 41.25litre fill up = 9.085
315miles on that tank = 34.67MPG
I've been working it out over several tank fills the last few weeks. (I drive 100miles a day for work).
As oringinally stated, my 2litre st did an actual MPG of between 29.5-32.5 hence my confusion?