While definitely not a modification to improve the looks of your vehicle, a pie tin can actually streamline the wheels of most vehicles on the road today. The problem with most pre-2000 vehicles (and many post-2000) is their wheels or hubcaps were designed with looks in mind rather than aerodynamics. The air gets caught up within the curves and holes, whipping around the wheel's wells, and causing a significant amount of wind drag.
Eco-modder has some great how-to ideas for boosting MPG, and this is one I found with a modest price tag of around $5.00 per wheel, and it has the potential to add as much as 1.5 MPG onto your vehicle. This could easily pay for itself within the first few tanks of gas.
The nice thing about this mod, is it can be installed and be completely reversible. This means that you can add the pizza tins anytime you are about to embark on a long road trip where you want maximum fuel efficiency, then remove them for your daily commute if you prefer. The only leftover hint of this mod will be three small holes in the center of the wheels, which can be covered with stainless steel bolts when not in use.
Here are the instructions to this project: