Typical generic MPG enhancers include but not limited to...
Make sure tires inflated properly.. Don't drive so fast Don't drive around with alot of extra weight in the car. Switch to synthetic oil Ensure the car has a tune up , good air / oil filters No dragging brakes fresh fluid in the tranny. Make sure your exhaust is not clogged.
Buy gas in the morning when it is cooler out , gas expands as it warms so you are getting less for your money when it warms up during the afternoon.
plan your trips
If the check engine light is on go get it fixed.
A car that is well maintained gets better mpg then one not taken care of.
Efficiency is partly related to getting the proper amount of air and gas easily into the engine and how quickly the end result can escape thru the exhaust pipes . Thats why motorheads go nuts over high flow exhausts and air intake systems
The rest is based on body aerodynamics, drivetrain design and upkeep, and the idiot behind the wheel.
Also don't expect miracles. Don't believe bold face lies for MPG add ons.