Subsequent to my earlier post in 2014 just after I bought the car, I have kept fuel figures from that day and each year print a graph for the consumption and pence /mile over the same period.
After my 5th year the figures are as follows-:
Years average - 44.57 mpg
Life Average - 47.32 ( this has dropped this year following the software change in 2018).
Max for year - 55.53
Min for year - 36.6
Median - 45.19
Total mileage 41K.
The life average is total miles/ total fuel bought.
The individual mpgs at fill up and the official figures, even world figures are at best very rough. All depends on how a car is driven, ambient temps, elevations, level of servicing, loading of the car i.e passengers , luggage etc.
You can be sure that any official figures are done on an empty car on the flat at its best ambient and with a very light foot.
Stating Urban, Motorway, and Combined are still guestimates, but driven for a year with mixed driving and weather gives a more accurate reading.
The cars consumption gauge does not actually measure fuel delivered to the engine, fuel returned to the tank, but based on sensors and can be +/- 5-10% out.
If the term 'filling the gaps' means what I think it means, then riding on somebodies rear bumper/ slipstreaming is a dangerous and bad habit.