Configuration 1 to 4 in 03 NASCAR and 05 NASCAR (GameCube)
Use config 2 for 03 NASCAR and config 4 for 05 NASCAR if you are emulating GameCube. The default config in 03 NASCAR uses A button for acceleration and B button for braking. Config 2 and 4 uses the R button for acceleration and L button for braking which means that you can use the triggers on your controller to play easily. Config 2 uses B button to downshift and the A button to upshift. Lastly, config 4 uses Right Stick (DOWN) to downshift and Right Stick (UP) to upshift if your using Manual transmission.
We're moving into 05 NASCAR. The default config is always config 1. Don't use config 1. The reason is that you downshift by Right Stick (UP) and upshift by Right Stick (DOWN). The best configuration for 05 NASCAR is 4. The reason why I'm saying 4 is that you can stop inverting the gears with the Right Stick and correct it by shifting up with the Right Stick (UP) and shifting down with the Right Stick (DOWN).