car consuming so much gas,whats the problem? on 1998 Nissan Altima

the sparkplugs and ignition cord as well as the fuel filter were replaced but still the engine consumes more gasoline.

Asked by for the 1998 Nissan Altima
Could be a stuck open thermostat, faulty coolant temperature sensor - these problems won't let the ECM to switch to closed loop mode, bad O2 sensor, wrong spark plugs, intake leak, bad fuel injectors, engine mechanical fault - low compression, etc....
I'd take the car to a good repair shop and let them to check it since so many different thing could cause this condition.