I'm working with a Wild Thumper 6 wheel chasis that is designed for use with an RC controller. However I'd like to have a mapping to a keyboard for control as well. Can you suggest a set of keys and behaviors that you've used to deal with the continuous value control normally offered by a joystick or pair of joysticks? The standard wasd keys + an accelerate/decelerate pair? I'd also take a pointer to a videogame that you think does this well.
Assuming you don't have other functionality that would take precedence, I would use WASD for movement and Left/Right arrows or mouse buttons for accelerating. Having the controls be divided that way mimics the conventional movement/action interface seen in many video games, while letting the user control the speed in parallel to motion.
However, ultimately, the right answer is whatever is most comfortable/intuitive. The above is what I'd try first, but the real best answer is getting user feedback.