Poor fuel economy.

Hi, I have Nissan Sentra 2009 and recently I measured fuel economy and I get only about 500km per fuel tank (hwy + city) which is 55 litres, while it should be doing 600+kms. It always seemed that car was wasting a lot of gas. I talked to the dealership and they said there is no way for them to measure the fuel consumption and offered some services like flushing the injectors (or something like that) which are quite expensive. I tried Lucas oil treatment, but same result. My driving habits are pretty good, I'm not an aggressive driver. The car is very well maintained in terms of oil change (synthetic every 8,000), if it matters at all. What do you think could cause this? IS there a way to check that mixture of gas and air is right? Can this be checked by perhaps connecting to micro-controller? Thank you.