I had a 04' frontier SC and it only got 13mpg so I added a K&N 75 cold air intake and that upped it to 16mpg and about 15hp, thatís really the only thing I would recommend everyone do to any car.
I would recommend K&N, they may be a bit more money but in my experience you get more bang for your buck, ex. more hp and mpg.
Most websites, like American Muscle and Latemodel Restoration, will have a place where you enter your cars info and it will give you options for your car.
90' SSP LX Coupe daily driver- 3in Anderson Power Pipe, Pro-M92mm mass, Explorer intake, GT 40P heads, 24Lb injectors, Lunati cam, BBK headers, Proform roller rockers, FRPP OE hydraulic roler lifters, 3.73, Griffin Dominator radiator, Mac x, Flow Master Super44's, and SCT 4-Bank Eliminator chip.