I get 15 average. Best was 16.5 on the highway with 35's and 4.56. You just need to do some checking and tuning:
New plugs and cap, proper tire pressure (I would try max pressure before doing the line test), air filter (no need for aftermarket, keep it stock and clean), check the vac system as mentioned, new fluids won't hurt but make sure you put 10w30 in the motor and and trans if you are running a manuel, not a heavy weight oil. Seafoam is great stuff and it is possible that your intake needs a cleaning. Dump it in the tank first, if you still aren't satisfied try the booster line and then change your fuel filter after this. There is no telling what crap will break loose in your tank from this stuff. Another thing on the seafoam, do it before you change any plugs or O2 sensors. Some say it can fowl them out.