[Fat Charlie]

You're burning gas to push a car across pavement and through air.

Most performance mods focus on how to burn more gas in order to push you across and through even faster.
Most ecomods focus on reducing the amount of energy required to push the car across and through.

If you want plug and play, low cost and stealth, then tire pressure and grille block are probably your only moves. If you're going to spend any money at all, get an SG or UG. They can be mounted unobtrusively, stay with you when you sell the car, and provide more real results than anything else.