Unlock the GT40 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, for a $35,000 to $45,000 profit.
Get a six gear car. Top it out with the maximum HP, ten test it out in the Sega Drag race. Make sure you get your car in a Manual Transmission. Start out with sixth gear, and accelerate just before the last second. When you go up to 40 mph, shift down to third and make your way up the gears.
Purchase any part/upgrade for any car and have the exact amount of money in your total money. You will receive the part/upgrade for free. For example, have $10,000 total money and purchase a Tuned ROM upgrade priced at $10,000. You will get the part for free with $10,000 still in your total money.
To get the nitrous from the used parts shop, you will have to lose at the drag races on a day ending with "5". For example, 305,15, etc. Return to the screen for the parts shop after the drag race. It will now be 306,16,etc. There will be a question mark at the end of all the parts, after the turbos. If your car has a turbo or if it does not appear, get out then go back in and it should appear.