did u ever stop to consider that the knock sensor may play a part in milage? or maybe the resonator box you removed? cars are built the way they are for a reason

i also mentioned drving style and altitude playing a part. if you'll notice the sticker that was on the window it said EPA estimated milage... means average milage on the cars they tested. you may have gotten a crummy milage car. there are so many different factors to consider for gas milage that it's hard to track down one specific cause. i'll start with the gas you use.
do you visit different stations or use different brands?
what is the octane rating on the gas you buy?
does it contain additives?
are you certain it's a respectable station?
is the gas oxygenated for the winter?
find a glass jar with a lid and clean it out. head to the station and fill up gas and ask about the oxygenation (supposed to lower emissions through adding more air to the fuel so it burns cleaner; means too lean for those with modified intakes or exhausts). also ask about additives then check with nissan about the specific chemicals they use for their additive. some chemicals have adverse effects with newer aluminum engine components. have the gas clerk put a little in the dry, clean jar and close the cap tightly. when you return to fill up for the third time, check the jar for seperation and water content. any more than 5% and find another station.
i could be wrong on some of my information so anyone with the facts please step in.