sling load,
The old balloon demonstration is a great way to help understand the concept of real vs apparant forces.
There is no significant pressure gradient in the air in the car, the balloon simply responds to the "bouyant force" which is an apparant force like "centrifugal Force". If the car were half full of water, you would see a slope as the water responded to the new "gravity field" that is the sum of the centripital acceleration and the gravitational acceleration. If we floated a toy moored mine in the water, we would watch the mine float the way the balloon does.
A bubble level does the same thing, of course.