Adjusting the nut behind the wheel is the #1 mod.
Getting fuel economy instrumentation is the #2 mod. It doesn't have to be a Scangauge - Torque Lite app ($0) + decent OBD2 bluetooth adapter ($22) = $22 instead of $90+.
P&G is the #3 mod. Works best with instrumentation.
Air dam is my 4th best mod, followed by tire inflation. Air dam will generally only help out a lot of the underbody of your car is unaerodynamic (most cars are).
Grille blocking and wheel covers helped some, but tested at a <1 mpg improvement for each at 40 mph. Wheel covers see greater benefit at higher speeds.
Of course there's always the boattail...but then again there are a few of us who still want to
use our vehicles.
