Realistic top down 2D car simulator using box 2d physics. This could probably help.
Where I last left off I couldn't figure out how to do the force transfer from engine RPM/torque to the driving wheels through transmissions gear ratio.
If that is not enough maybe a 3d first person projection similar to Ultima Underworld, Doom or Wolfenstein 3D by not using Q3D plugin. Rojohound has done some 3d platformer stuff with Paster plugin.
Probably not what you were after but, there was no other challenge suggestions