As above you need to be a bit more scientific. Clearly you can't rely on your fuel gauge so you need to fill your tank to the top, record the km and then next time you fill it calculate the litres per 100km. What are all the things you did to improve economy? New spark plugs and coils? Check brakes for binding? Check timing? Did the mechanic put an AFR meter on and if so what were the numbers (or do you have a wide band installed)?