I take it these are not U.S. mpg: I got excited for a while there.
UK gallons are bigger than US gallons:
1 UK Imperial Gallon = 4.55 litres
1 US Liquid Gallon = 3.79 litres
2) - UK mpg figures will be calculated using the EU's methods, the US mpg figures will be calculated using the EPS's methods.
Also use different cycles (Urban, Extra-Urban and Combined for the EU, City and Highway for the EPA) don't directly compare.