Try typing the following during the game for interesting results:  
  
aerf     - Fast acceleration and quick braking (and free life)
           Keep retyping it for lots of lifes.
gass     - Jump to gas station with amazing average time and score
gasst    - Jump to gas station and use real average time and score
,bruce   - Change sign on level 2 from 'Play Grand Prix Circuit' to
           'Bruce Dawson Wrote This'  
6outran  - unknown