If you don't want to have to cut down on the level of detail in Sega Rally 2 but would like the frame rate to remain at a constant 30 frames per second, enter this code at the title screen: Up, A, Down, Down, Left, Right, B, B and Up.
This is an addition to the game that lowers the detail level and speeds up the game's frame rate: Up, A, Down, Down, Left, Right, B, B, Down. You'll hear a sound when you do it correctly.