Complete some periodically throughout the story, or save them all for the endgame grind - whatever works best for you. This can be time-consuming, so take things at your own pace. This logic also applies to your knowledge of the city the better you know the streets, the easier it will be to traverse them! As such, you may want to complete all Driving Schools Missions prior to beginning many of your fares. As such, it is still recommended that you cut corners and use shortcuts as much as possible.
Though this all seems daunting, there is a decent amount of give in the time allowance and acceptable severity of your driving.Below you can see the accrued time, the number of fares completed, and the tip bar. The tip meter acts as a risk-reward system, which will reward you for delivering fares quickly, yet punish you for driving carelessly OR for taking too long. San Andreas is the first GTA game to add a tip meter to taxi faring.
