I have bad gas mileage and I drive the speed limit ,if not less on my car
Chrysler Sebringlx 03..it has high miles 146k..It doesnt even seem like 18mpg mixed driving!

 

Uh, wrong brand of car for this forum.

 

well first and for most, when was the last time you changed your fuel filter? almost positive it is internal... next would be plugs/wires and air filter.... i would also look into injector cleaner

some in the tank and some through the vac system, but as for a tune type thing for your car... i wouldn't mess with it... for the price it wouldn't be worth it and will wear more on that motor and chrystlers are known for issues