your car 20 euros / 1.49 = 13.42 litres of fuel. 13.42 / 4.54 = 2.96 gallons. 120 miles / 2.96 = 40.5 mpg.
Girlfriends car 200/2.96 = 67.59mpg .
TBH 40.5mpg doesn't too bad if you are doing local trips . The A6 is a big car what kind of MPG are you expecting.
Your girlfriends car is awesome, when you find out what has been done to that please post on here so everybody can have 1.0 litre diesel fuel economy.
Being serious there are a million and one things that can cause a car to use more fuel, tyre pressures, cheap tyres with poor rolling resistance, sticking brakes, engine issues ( Turbo boost leaks, cam timing, Coolant temperature sensor fault, MAF faulty, VNT mechanism sticking, thermostat faulty, etc etc).
Karl.