I have a slightly different view of modifications.
This largest factor is how hard you press the gas pedal.
If you look far down the road and judge where and when to react with changes it saves a ton.
Also most all power adding modifications are to free power from centrifugal or vacuum forces so it can be used to turn the wheels.
So by reducing restrictions in the intake and exhaust , ensuring good combustion for a complete burn and having maximum air pressure in your tires to reduce rolling resistance. All of these things together will help you save tons of fuel.
FYI i have a full cold air intake with a ram pipe forcing air so it runs slightly lean , a full exhaust with X pipe. Lowered more than most , new iridium spark plugs , motul oil ($100) not run of the mill mobil1 , removal of the viscous clutch to an electric fan , Leds for all non braking lighting for reduced voltage , almost perfect alignment and a good coat of wax.
All of these things help bring my car above the standard for V8's never mind LS400's