Before we advise you to do something that might get you into trouble, we need to know a little bit more about the relationship between your father and your car. Does your father drive your car frequently ? Is it more "your car" or more "his car" ? If it is more his car than yours and if he drives it frequently then Ide tell you the only thing you can do is probably small things he wont notice much or object to. The following mods may improve gas mileage while adding power.
If he does not drive the car much, Ide say add some 3.73 gears. They're the perfect gear for the 96 GT. He wont see them cause they're in the differential but you can definately feel the acceleration gain with them. You may loose some gas mileage but on some cars people actually gain gas mileage in town with a steeper gear.
K&N FIPK(although he will notice this if he looks at the motor frequently but it may help gas mileage) If he disagrees with this mod, just get a K&N air filter and slip it into the air box. He wont notice it unless if he opens the air box to have a peak.
Steeda u/d pullies. Unless if he really looks closely at the motor, he wont even notice them. They will free up more power and torque and can increase gas mileage.
Magnaflow cat-back. I recommend magnaflow because of its wide open technology lets exhuast pulses flow smoother and thus makes more power. This may increase your gas mileage then again it may not. Although it will give the cars exhuast a somewhat louder sound, it wont be extremely loud like flowmasters or Borla would. Unless if he listens to your car all the time, he probably wont notice it.
Colder spark plugs and wires. Although this wont really add much power, you can pick up some gas mileage with it and get a better ignition response. Unless if he checks under the hood, he wont notice the wires. To keep him from spotting them, get some that are black. He probably wont tell the difference. The plugs he wont see naturally cause they will be in the motor.
Sport chip - With a sport chip, you can calibrate your speedometer for a gear change and advance your ignition timing. The chip can be seen if he looks under the drivers side dash. If he does get in the car much, he may not notice it.
Aside from these mods, I dont know what else to tell you. Try and work something out with your father and if the car is more his than yours it would probably be unwise to do anything he didnt approve of. Good luck and let us know what you think of our suggestions.
