Gas Milage?

Do a full tune and compression test, fill the car and drive 100Km's or as close as you can and re fill this will give you and accurate litres per 100 Km's or miles per gallon depending on what you deal with, do this and see what the readings are, tank gauges are just a guide and not at all accurate for economy testing.

What part do you not understand?

You need an engine compresion tester, this screws into the spark plug thread and you then crank the engine 3 or 4 reveloution on the key and read the presure reading, you must take all plugs out and earth the coil lead so you do not get a shock.