Enter codes under "Settings" -> "Cheats".
Enter IRONMAN as a code.
Weaker police cars:
Enter KEYSTONE as a code.
Enter ROLLBAR as a code.
Enter CARSHOW as a code.
Enter ZOOMZOOM as a code.
Enter GUNBELT as a code.
All weapons from era:
Enter GUNRANGE as a code.
Enter TOOLEDUP as a code.
Enter FLIPFLOP as a code.
Beat the game to unlock the Atlus Racer, Ram Raider, and Negotiator Vehicles, as well as the "Then and Now" Era Change option.
To get parts cheaper, unlock the Era Change option by beating the game. Then, use inflation to your advantage. Run all your side missions in 2006, as they net you more cash. Then, buy all your parts in 1976 for ten times cheaper.
Choose to enter a circuit race. When in the difficulty selection part, drive your car to a garage and pimp it out. Then, start the race on any difficulty, and when the race starts, pause and press "restart". You'll be outside the track with all your new parts and your money refunded.
Get your odometer to 666, then melee attack a pedestrian to take control of them! Spooky!
Night Night mode:
Get your odometer to 700. This mode makes it nighttime, all the time.
Shortest Day mode:
Get your odometer to 800. This mode makes a day last 24 seconds.
Far Out mode:
Get your odometer to 900. This mode enables a tripped out psychedelic effect. Groovy.
Collect 10 star tokens in either era.
Collect 20 star tokens in either era.
Collect 30 star tokens in either era.
Double V-edit car durability:
Collect 40 star tokens in either era.
Free vehicle upgrades:
Collect 50 star tokens in either era.
Thanks to Revolution Readers m6, DEBRA, HarMar, p00fs, BurgerTimer!