unfortunately any aftermarket intake will decrease mpg same with many other mods, 25-28 isnt bad on a civic, u sure u got that much b4? advancing and retarding the timing can have unpredictable results as far as gas mileage goes, it depends on ur driving style on which way would yield more mpg, unfortunately with modifications done to ur engine expect to get worse and worse mpg. but our little hondas should always get a good 22-25 mpg as long as uve still got a n'a setup no matter how bad ur setup may be