From GTPlanet Game Guides

Quick tune is a feature in Gran Turismo 5: Prologue, similar to the "Settings" feature in GT4, but with less options (as GT5:P does not have the feature of buying parts to upgrade performance on your car, and therefore, any settings related to that were not needed.)

Online

Quick tune is used Online for Determining Performance Point value and car setup, it is only available in intermediate, pro and expert class races, but it not allowed in beginner (due to drivers in beginner not having access to it until they unlock "S" class)

Offline

Offline, the feature is used for the "S" class races, as well as other modes, in some modes however, the options for tuning are limited.

Setups

There are multiple setups on the GT5:P Car Tuning and Settings forum.