Hi! i don't know what kind of car you have, but in some cases performance mods(when done right) are good for mpg, i'm one successfully case of that and i've done other good works in other cars from my clients, my car stock was about 6,5L/100 now it is 5,1L/100 and it's way faster and safe and i dont need to care too much about driving to get that.
One thing about performance, the idea of tuning is to set you vehicle to get the most performance from the fuel you inject(of course that if you inject more fuel, it will be faster but also will decrease your mpg, air only don't create horsepower, you need fuel to burn heheeheh), if you get the best, when you're not pushing too hard your fuel economy will be good.
you can also tune weight, aero, suspension(if your car has good handling you dont need to be so hard on the brakes).
gearbox and clutch should work fine, wheel alignment, good flow on intake, etc, etc.
If your car is a tdi feel free to contact me, i'm from europe but i can provide you good information, i work a lot on this engines(my english is not so good as my mechanic skills
)