First and last thing I'll say on the matter:
MPG is useless without context. Context in this being: how much is fuel and how many miles do you drive.
You can use sites like http://www.petrol-calculator.co.uk/ to work it out quickly and easily. Where it says 'Pence/L' or has pounds, put in cents and pretend it has a euro symbol.
Example: you do 10k miles, 40mpg, 1.60/litre, it costs EUR1818.44: http://www.petrol-calculator.co.uk/?d=10000&c=40&p=160

MPG is often a factor, but it should only be used in context. If you only do 5k miles per year and you're tempted to change car because the current one does 30mpg but a new one does 40mpg, ask yourself if 300 euro a year is worth buying a new car for.