I would say that the OEM engine tune is sufficiently developed to be as lean as possible relative to the fuel octane.
And so improve fuel economy by increasing the tire pressure. Also, shorter sidewall tires are possible. For instance a 205/50-15 tire and a 205/45-16 tire have about the same diameter and width but the 205/45-16 tire has a shorter sidewall.
If a manual transmission, then improve fuel economy by not lugging the engine and by not highly revving the engine. Shift the transmission to be in the correct gear at the correct time. Otherwise get an automatic transmission.
Now a front air-dam will improve fuel economy but a front air-dam can have a larger effect on reducing aerodynamic lift at the front than at the rear and therefor there is a risk of a high-speed aerodynamic oversteer. So current technology more often uses a front splitter and then smoothing panels on the underside of the car.
A rear lip spoiler will improve fuel economy but only have a small effect on aerodynamic lift at the rear.
Strangely enough a stiffer suspension will improve fuel economy because a stiffer suspension produces faster weight transfer and faster weight transfer results in less tire drift in ordinary driving. (Tire drift is what a tire does when it is not sliding. The sport of drifting is really the sport of power-sliding.)
There's no easy answer in exhaust systems. Current cars mostly need a tune just to use a low-restriction cat-back exhaust. For instance, popping sounds from the cat-back are due to un-burned fuel in the exhaust system. At least the electronic tuner then has something to tune-for.
Easy choices, but expensive choices, of vehicle weight reduction are available in AGM batteries, with premium racing style wheels, and in stainless-steel exhaust systems. The AGM battery can simply be smaller than the OEM gel battery.