bad gas mileage

If your fuel economy is extremely bad, you were right to look for air leaks. Continue to look for air leaks and vacuum leaks. The intake manifold could have a vacuum leak. You could have a heated oxygen sensor operating sluggish - causing too much fuel to be dumped. Take a look at your thermostat - is it staying open too often and not allowing your engine to get hot enough often enough? A cold engine burns more fuel. Do not forget that there can be other variables that can cause poor fuel economy - drag coefficient, short trip driving, etc. Good luck!