1999 Hyundai Elantra GAS MILEAGE

I HAD RECENTLY REPLACED THE HEAD, GASKETS, SPARK PLUGS AND WIRES ALONG WITH A FEW OTHER THING DUE TO THE ENGINE GETTING HOT AND WARPING THE OLD HEAD. AFTER ALL OF THE THAT THE CAR SEEMS TO RUN GOOD BUT I'M ONLY GETTING A LITTLE OVER 20 MPG WHICH IS QUITE LOW FOR THIS CAR. ONE THING I NOTICED IS WHEN I REMOVE THE GAS CAP ON A LOW TANK IT'S NOT RELEASING ANY PRESSURE LIKE A CAR NORMALLY WOULD. I DID REPLACE THE GAS CAP BUT IT DIDN'T MAKE A DIFFERENCE. ANY IDEAS?