To get a lot of money, REP, and Speed cards quickly, complete short events. Since all events give almost identical rewards regardless of the difficulty, just keep repeatedly completing the shortest events. The fastest event in the game is the "Drifting The Block" drift trial. It becomes unlocked early in the game, when you do drift trials for the first time. It is the perfect farming spot for money, REP, and Speed cards. This event can be completed in 90 seconds, and since it is a circle, you finish at the start and can instantly replay it. Each time you finish the drift trial, you will get approximately 12,000 credits (17,000 credits if you sell the Speed card), 120,000 REP, and a Speed card. You can get approximately 3,600,000 REP and 510,000 credits per hour using this method.
Keep replaying the "Hyperspace Circuit" race. It becomes unlocked during the final act, at the end of the game. It gives 31,000 credits (if you sell the Speed card), 144,000 REP, and a Speed card. The event only takes three minutes to complete, including loading times and transition screens (with a fully upgraded car). It also immediately puts you back at the start of the event. You can get approximately 2,880,000 REP and 619,980 credits per hour using this method.Unlock Derelict cars
Collect the five hidden Derelict car parts for a Derelict car to unlock it. Each Derelict car requires you to have defeated a different league boss. After defeating a league boss, you will get clues for the five Derelict car part locations of a certain car. In the map menu, select the "Derelict" tab to view the clues. It shows a drawing with outlines of the streets where the Derelict parts are located. You normally would have to look at the in-game map and compare it with the drawing, but the videos below show all the Derelict part locations. The Derelict cars are special because they can be upgraded to the maximum level without the level caps normal cars have. After upgrading a Derelict car to Performance Level 300, it unlocks a Super Build for more extreme visual customization. After restoring a Derelict car, you can also choose which category you want it to be (Race, Drag, Drift, Runner, or Offroad). It is recommended you choose a different category for each unique car. None of the Derelict car parts are missable, but they will not spawn until you have defeated the corresponding league boss.
Chevrolet Bel Air 1955
To collect the five Derelict car parts for the Chevrolet Bel Air 1955, you must first complete the Runner quest line.
Chevrolet C10 Stepside Pickup 1965
To collect the five Derelict car parts for the Chevrolet C10 Stepside Pickup 1965, you must first defeat the "Riot Club" (Big Sister) league boss (Drag quest line).
Ford Mustang 1965
To collect the five Derelict car parts for the Ford Mustang 1965, you must first defeat the "La Catrina" league boss (Race quest line).
Nissan Fairlady 240ZG 1971
To collect the five Derelict car parts for the Nissan Fairlady 240ZG 1971, you must first defeat the "Udo Roth" league boss (Offroad quest line).
Volkswagen Beetle 1963
To collect the five Derelict car parts for the Volkswagen Beetle 1963, you must first defeat the "Shift-Lock" (The Underground Soldier) league boss (Drift quest line).
Drift for eight seconds to unlock the Front Bumper visual customization. Buy a drift car, and go to the airport. It is easy to drift for eight seconds on the runway at the airport.All billboard locations
Go to the indicated locations to smash through all 30 billboards. Each billboard needs to be smashed through using a ramp. Most billboards do not require powerful cars to reach -- so you can do this early with cars under 200 power. The 30 billboard locations are split into three sectors. They are collected from top to bottom, going through the map from left to right.
The following video shows all 30 Jump locations completed with 3 stars. The Jumps are challenges across the map. Most of them can be completed with 3 stars early in the game with cars under 200 power. A few require a car build focused on jumping and air time. The Porsche Panamera Turbo is recommended for the harder challenges. Try to get your Air Time skill up to 9 by trading in or buying speed cards to easily complete all 3 star jumps.
The "Eject Button" Jump challenge is one of the hardest jumps in the game, and requires a car that is made for jumping and some practice and luck with the ramp. A good car to use is the Porsche Panamera Turbo with the Offroad build. Mainly go for jump speed cards. These will increase your airtime, and you should get it above 9 or even close to 10. The "Eject Button" Jump is especially difficult because you have to hit top speed while going against traffic and squeezing through a narrow spot to hit the ramp. This takes some practice.