This seems to cover just about anything you can do to make your car faster -

* Any wheels
* Any tires (including R-Compounds)
* Any strut tower brace or cross-brace
* Any cat-back exhaust
* Any air filter & air box
* Any carb
* Any brake pads & rotors
* Any spoilers or aerodynamic aids
* Any seats or steering wheel
* Any aftermarket chip

And that just puts you in Prepared. What are some examples of things that would vault you into Modified?