If the road is flat enough, you can use an homography in order to solve your problem
You told us knowing 8 correspondences world / image. Using these, compute the homography from image plane to road plane. This 3x3 matrix will give you the mapping from image to world (but only for points belonging to road plane, which is a true assumption for the wheel/road contact). Use cvFindHomography or any approach you like
I guess that in this case, you don't need the internal param anymore
Then you can move on with your task...