Hi, Rejnman; There are sooooo many things that can affect fuel economy. Here, (USA) the Fed. gov't makes the fuel companies ad ethanol to our gas during the winter months which causes about a 2mpg loss. Ambient temp also can cause a drop in fuel econ. Tire pressure is very important, also. Synthetic oil can help. Don't forget the air filter. Have you ever had the fuel injectors cleaned? That alone will help.
You didn't mention spark plugs (assuming this is a gas engine).
I am driving Ford Mondeo 2.0 16v 145hp, year 2004. I had problems with compresion, there was no compresion in one cylinder and my fuel consumption was waaay to high(30l/100km). Now I resolved that issue. Changed some parts and now its fine. And I also changed a lot of other things (both belts, water pump, all fluids/oils, oil filter,battery, alternator...). After all of that car is going great. Dosent twich, starts great even in cold weather, has a lot of power. Its drives like its brand new car even though it has 165 000km.
But after a few hunder km I realized my fuel consumption is still betwen 25-30l per 100km and thats even on open road(it should be around 7-
What can be cause of such a massive fuel consumption even though car is running so smooth?