turbo will increase the air pressure you intake. thus, since you have a higher air pressure, there will be more combustion output than what youd normally get. this means you get an increase in horsepower, so it will take less to reach certain speeds and maintain highway speeds. overall, youll get more out of your engine, but mileage wise its almost negligble. some engines, however, increase fual pressure to correspond with the added air pressure, which means you burn more gas //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif you get more horsepower, but at a price.....
a chip will tweak out your fuel/air intake/exhaust ratios, making your engine much more efficient. youll gain horsepower, and with this mod youl gain an increase in mileage. coupled with a great cold air intake system, and something like a dynomax exhaust system, youll see a vast improvement overall on your vehicle.
btw, you can either get a chip that will force the cpu to keep the fuel level the same even if you add the increased air pressure, which seems like something you want, then there are others that increase both fuel and air pressure, with or without the additional mods. best thing to do is ask a mechanic at your local dealership. just asking for advice is free, just dont let him sell you anything //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif