Jesus H.
Miles "to a tank" - when do you give in and fill it up again?
Miles to the £ - how much did the fuel cost?
Back in '02 I'd have got from here to Belgium for £50, now I might just make Brighton.
Equally, if I fill up the second the gauge moves from its stop, Id get 50 miles "to the tank".
Accurate MPG calculation isn't the thing of genius, but it does require a modicum of intellect, reseting a trip computer and some actual maths.
Brim-to-brim, how many litres?
Convert to gallons (4.56? / Imp Gal?)
Miles covered / gallons = "real" MPGs
Every other way is less accurate, like a 200hp ph2 that got its figures from an iPhone app...and an eBay chip