There are not any "gadgets". I would recommend you have your fuel injectors cleaned, check tire pressure and alignment. If you have over 100K replace your oxygen sensors. You can also change all fluids to synthetic and upgrade to a performance air filter (K&N) and performance exhaust. Of course after spending at least $1000 you have to weigh how long it will take you to make that back from the increase of 2-3 MPG!
thanks for the tips and good advice. an increase of 2-3 mpg may help and i will be willing to do it in steps, after all changing fluids to synthetic goes along with an oil change; i can add fuel injector cleaning agents to gas tank; tire pressure is free, alignment is not necessary at this point, etc. a performance exhaust is a great idea! hadn't considered but will save towards it. it passes smog so the oxygen sensors must be okay?