Test your car's performance any way you want with Race Timer. Set your own checkpoints (in Placement Mode), and take any car through the race you created.

As you drive, Race Timer shows you at each checkpoint:
Elapsed time
Fastest split time: delta between your current time and the best time previously achieved in any vehicle
Vehicle split time: delta between your current time and best time previously achieved in the same vehicle

Installation:
Place LapTimer.dll in your scripts folder.
Latest ScriptHookVDotNet is required.

Placement Mode:
In this mode, you will create your custom race by placing checkpoints. Enter "Placement Mode" with F5.
Ctrl+X: place new checkpoint
Ctrl+Z: undo last checkpoint
Ctrl+D: delete all checkpoints

Race Mode:
Once you've placed at least 2 checkpoints, get in a vehicle and press F6. You will be teleported to the first checkpoint, and the timer will start. Times will be displayed at each checkpoint and at the end of the race.
Ctrl+R: restart race