Well.
I Ignore the Trip computer MPG numbers. Those are wrong 100% of the time.
If I fill up then do just motorway 70MPH and then fill up again i get 37-38Mpg, Depends on how many hills are on that stretch.
if I fill up then do my Daily commute which is 22Miles each way, 15mins Motorway/Fast A road, rest town I will get 32.5MPG
If I fill up and just do town driving I will see a return of 26-28Mpg.
I work it out at the pump, and IMO if you cant work out the MPG going off the Pump and Miles done then you need to sell up and live in a hole.
But if you want to work it out then.... Fill up. Take the L just filled and divide by 4.57 you will be left with the Gallons.
Then Divide Gallons by the Milage. You will have the MPG.
So for example: 320 Miles, Filled up at 44.35L
44.35 / 4.57 = 9.70
320 / 9.70 = 32.98MPG