Thanks, Namuna!
I should be finished going through the 9XXX codes by tonight.
So far, I haven't found any codes that do any tricks or stunts.
Many of the codes seem to be custom settings for the speed and steering. Maybe some of the codes let you imitate other cars, like making your Skyline run like a Beetle, but it's hard to tell.

I'll give another progress report by tomorrow (Saturday)