1996 Honda Civic gas milage drop after adding muffler.

i had recently installed an additional muffler on my car. no change was made to the existing stock exhaust lines except the connection point.it was cut and the new muffler connected in parallel,like a mirror of the old one.connection was made about one meter before the muffler inlet. now engine and exhaust is more quiet, i also felt a noticeable increase in power or maybe its just my feeling.also my gas milage went down from 15km/liter to 11 or 12km/liter. i calculate gas milage based on distance traveled in a full tank divided by the amount of gas in my next refill. i experienced b4 milage of about 13km/liter only when im always speeding and needed fast acceleration. i thought giving my exhaust more room to breath would make it save more gas. or maybe im having another engine problem. tnx in advance for the help u can give.