This is the way i work out MPG:
Total number of litres in a brimmed tank converted into gallons.
Total distance driven (before flashing light comes on) divided by total number of gallons in full tank.
53 litres in full tank converted into gallons = 11.6583 gal(UK)
Total distance driven before flashing light = 365 miles.
365 divided by 11.6583 = 31.30 MPG
So i figured i'd try this calculator
Under 'real life miles per gallon'.
Fuel cost: 122p
Total spend: 53 litres
Distance: 365 miles
^^That is what i did last time i filled up. The calculator came up with:
Your real-world MPG average is 38.20, covering 365 miles using 9.56 gallons of fuel.
How does that work?
Also, if the Astra Mk4 has a 53 litre tank, why does it never cost me more than £54 to fill it from the flashing light, surely you are down to fumes when he light is flashing?