Earlier quoted context omitted.
Probably fairer to compare your Mercedes to the same model with a diesel engine, and likewise your Nissan. Wiki says the 1.6dCi is available in 130-180hp whereas the 1.6 petrol has 107-116hp Fuel consumption will vary due to a lot of different factors, such as aerodynamics, transmission, driving style etc.
he probably means its not that big of a difference for him. and sure, if you dont drive a lot then it does not matter. when i drove 30k per year i drove a diesel, now im driving 15k per year and drive a gasoline car.
My diesel Peugeot 306 1.9 did about 45mpg no matter how hard I drove it, whereas my friend with the same model with a 2L petrol engine said he was using roughly twice as much fuel as me. The later common rail 2L version was supposedly capable of 55mpg