ALL THE CODES
These should be entered as the driver name in the Create a Driver screen:

3/4 gravity- FEATHER
1/2 gravity- FLOAT
twice gravity- STONE
better gravity- RADBRAD
no car collisions- BANZAI
just wheels- WHEELS
fat wheels- FAT TIRE
as though you won- WINNER
invincible- IMMORTAL
two lap races- GEK
dual-analog controls- PUSHBUTT
sunset tracks- SUNNYSKY
night tracks- NIGHTRID
grid track(s)- EPILEPTI
space tracks(s)- SPACERID
hidden paint styles- ROOSTER
ENTER MAXCARSn, where 'n' can be 'A', 'B', 'C', ... where 'A' is 1, 'B' is 2, ... it will be the number of cars in the grid when you race in the game. So, if you just wanted to race 4 other cars in one-player, you would go into create-a-driver and name yourself MAXCARSE (4 + yourself = 5 cars which is 'E').
Exclusive code for Dual Analog Controller
If you go into create-a-driver and make your name PUSHBUTT, for the dual-analog controller, if you push down (by that I mean push the stick INTO the controller) on the left controller you will get the reverse view, and if you push down on the right controller you will get the clutch. Enjoy! :)