We are moving cross country. And was looking at a car charger for my wife's macbook 12" retina. I'm guessing the standard USB accessory would be insufficient.
As with most other USB-C car chargers, that appears to only give 10A (5Vx2A) to the 12" MacBook. If the MacBook is in use, the battery will actually drain slowly, as it won't keep up with the battery consumption. If the MacBook is sleeping or off, it will trickle charge and take several hours to top off.
The only one I've found that will allegedly give the full 29W (14.5Vx2A) just like the Apple provided adapter, but for the car, is this item: