Hi,
In order to get some decent control over speeds etc. in a motion path, you need to rebuild the curve so that the parametric value increase at an even rate throughout the curve. Then by keyframing the parametric value of the motionpath you can get the car to do the desired things at a predictable rate.
Another way is to "drive" your car of course. I.e. create some attrbutes like "linearSpeed" and "rotationSpeed", set keyframes on these and then use an expression to drive the car. The drawback with this approach is that the timeline cannot be "scrubbed", the scene must play each frame in the correct order otherwise your car will drive off into the sunset.
The expression should be something like:
ry = ry + rotationSpeed;
x = x + linearSpeed * sin(deg_to_rad(ry));
z = z + linearSpeed * cos(deg_to_rad(ry));
The "driven" car ought to be possible to bake into a motionpath in the end though... haven't done that but I guess it would be possible.
Anyway, one of the most important things to realize though is that the car's pivot-point must be between the rear wheels. If you think about it you'll figure out why - its just the way the vehicle behaves.
Cheers!
/ Daniel