This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
(This trick may not work in Rad Racer II, but it reportedly works in
Highway Star/Rad Racer.)
Using the tachometer trick in article 6.10.2, try getting to the demo
screen and pressing the B button at least 60 times.
Then, hold up & right and press Start. You should see the game's ending.