[Yaristock]

Saggin Wagon

Interior
Remove excessive sound-deadening
Replace electric windows with manual
Replace all bulbs with Led bulbs

Body
Use sawzall to reshape rear into a teardrop shape ending, utilizing the stock rear windshield(to reduce costs, also replacing with plexiglass.)
Rear wheel skirts and flexible front wheelskirts
Grill Block
Underbody cladding
VW style air diverters to divert air from wheel area at the bottom
Shaved door handles
A type of air deflector to cover the area where the wipers reside

Engine
Increase EGR size
HAI
ECU remapping
Composite intake for weight savings
Reduced injector size
Engine Block Heater

Suspension
Reduce right height by 3inches
tighten suspension to reduce excessive bodyroll(possibly resulting extra engine load)

Wheels
Lightweight allows and lightweight wheels to reduce rotational mass

Brakes
Stainless Steel Lines
Powerslot Rotor
(saving gas is worthless if you die)