A car is not a Character. You should try to use Vehicle instead.
Also, the AIMoveTo functionality in Character is not suitable for a driving vehicle. It will turn in place and do other very un-Vehicle things.
If you absolutely need to use Character, then make the capsule of the character big enough that it encompasses the entire car. You may need to sink the car mesh down from the center of the capsule to make it meet the ground if you do this. Then set the navigation agent radius in your navigation mesh volume to whatever the radius of that capsule is, and re-generate the navmesh.
But, really, you should develop a vehicle AI agent that knows how to turn/steer; that will give you much better behavior in the end.