Nitrous Oxide:

Lose a drag race on a day ending with "5" (5,15,35,105). Return to the parts shop after the race - it should be a day ending in "6." There will be a ??? at the end of all the parts - look there for the Nitrous Oxide. If your car has a turbo or if it does not appear, get out then go back in and it should appear. To use during a race, press Y.

Quick money:

Unlock the GT 40 Concept car from the Season 1 Official Races. You can buy it for $150,000 then sell it for $185,000 to $195,000. It only takes about one or two game days to sell it, and you make a nice profit.

Faster drag race:

During a drag race, turn your TCS (Traction Control System) off at the options menu.

Control replay:

In "main view" mode, use the Left Analog-stick to move it and L and R to zoom in and out.