The Car Swap allows the player to change the car he's using in 'The Run', to any car in the game he has unlocked.

Meaning you can now use a Tier 5-6 car at nearly every stages of the game. Nearly means you can't use this glitch at anytime.

Explanation : Any PC version of the game should work. You'll need to start a new 'The Run', this glitch doesn't work through the stage selection. When you'll be on a stage that allows the glitch. (still needs to be tested) Finish the stage. Retry the stage from the end race menu. Quit the stage when in the race again. Go to the Challenge Series. Select any stage that contains the car you want. (only challenges from Stage 9 & 10 contains Tier 6 cars)

For example : Select 'East Coast Challenges' Select 'Drive Hard' or 'Drive Hard with a Vengeance' Thoses are two events to include the top-end Tier 6 cars

Select the car. Launch the Challenge Series event. Quit the event. Resume The Run.

You'll now be driving the car you've selected in the Challenge Series event.

The gain :

Using this glitch will result in a loss of a minute and 15 seconds if done well. Minus the car change for the VW Golf Mk.1, you got a minute net loss. By driving just carefully to not crash, you'll pick up the time you lost with the change quite quickly on long streches without car change (Stage 1-4 / Stage 5-8).

source, visual tutorial :