Maxima SE 2002 Upgrades (Fuel)

I am purchasing a Nissan Maxima SE 2002 in March and was wondering what upgrades I should install to increase fuel effeciency without a decrease is power.

I know this is a pretty lame question but I am not familar with car modification... I am learning (started today).

Edit: Also... how is horsepower measures?

Edit: Again... when installing a y-pipe does it replace the b-pipe or connect to the b-pipe (diagram would be nice)

I'm asuming you want to know how HP is measured. You can put the engine in a dyno or use a rear wheel dyno. There is a complacted formula that is used to calculate the HP but I don't remember it right now.

Just wondering if you guys know, what kind of acceleration do the 2002 Maxima 6-speed Se's get?