In this case, I can just tell you what I DID.
Start tracking your fuel economy and logging it with the garage tool on this site.
Read the hypermile driving techniques and start to "adjust the nut behind the wheel" as people here like to say.
Ensure that your car is properly aligned and your tires are filled to max pressure.
Consider some of the modifications you might eventually do by reading as much as you can in the Ecomodding/Aerodynamics section, but hold off on doing anything that costs money until you are reasonably certain that it will pay for itself.
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