mods haven't affected my mpg's, and never a mod induced mil light. bought the car new, it got ~32mpg. it now has 112k and my last few tankfulls netted me ~33mpg. this is with easy freeway cruising. YMMV
all you guys who have ecu's shouldn't see any mpg degredation. but jwt doesn't make an ecu for me. that being said, i have seen my mpg slowly get worse. until jwt makes my ecu (please), i'll just deal w/ 250 miles to a tank.
99 Sentra SEL platinum gold w/ HS headers and CAI, custom b-pipe, arospeed exhaust, magnecor 8.5mm wires, eibach pro-kit, courtesy nissan frt and rear STB, smc short shifter, greddy knob, jwt clutch, k&n crankcase breather, 17 deg. timing, clear corners and taillights and not a single stock bulb.
I got about 35mpg in my black car. It was modded with s4's,jwt ecu,hs 3rd gen header, 2.5 catback, pop charger(steallen), and a LEAD FOOT BEHIND IT!!!!
But now I have a blue one and I got about 42mpg on the way home from picking it up in wisconsin. I only had to fill up a couple of times on the 1000 mile trip home. What a wonderful thing!!!
If anything I think I gained gas mileage in my modded black one compared to stock. Good luck with your decisions on modding.