I do mine with an XL file, amount of fuel(brimmed every time) and mileage covered, XL then works out average MPG at each fill (and other useful, to me, information).
This is the summation at the end.
Total gallons 68.32
Average MPG 41.39
Total miles 2828
Cost/Mile 14.10 Pence per mile
Total fuel expense £398.87
Bought the car 29th May 2019, last fill 22nd Sept 2019, 2.0L CDTI auto
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