Great timing with the April update giving Android Auto access to the complete screen real estate. It looks like widescreen is still a multi-pane approach. I really hope you can swipe to collapse the two smaller panes to give nav the full screen when using the car radio.
Found an interesting Android Auto bug: Added a few tunes to what I keep on my phone and removed some I no longer wanted. The old ones stayed in AA on the car and the new ones did not appear. A reboot of the phone resolved the issue.