are you using the physics engine for the vehicles?

i am not sure what exactly you want to achieve but you could have a look into the python version of knights on wheels (follow the python wrapper link in my signature). it features a simple autopilot mode now which doesn't work too badly considering that it's only a few lines of code.

my racing tracks contain a path and i simply look 3 nodes forward to steer towards that node and i look 10 nodes forward to check if a curve is coming and brake if necessary.