hey i think i am getting really low mileage on my car.....

just serviced it and they say there is ROCKER COVER GASKET LEAKING.......
can't see my fuel light on when fuel is nearly empty and i am not getting a clue to find out how much kms i am getting per ltr...
when i open my fuel tank it is like opening a shaken coke bottle...can hear the gases coming out....
think digital meter is not giving right information about fuel consumption , how much fuel left, kms/ltr and other....
what should i do....is that all because of that gasket leaking or there is other problem going on.....