I've had my bike on the back of a car for a few hundred miles in the rain and I've always rode in the same conditions + grit, sand, and gravel dust. I wipe off my chain and apply a fresh lube of choice and go about. The only thing I do is lift the front of my bike vertical and bounce it on the ground on the back wheel to get any collected water out of the bottom chain stay. My drain holes are back by the dropouts. Water seepage if any into the vicinity of a greased bearing does absolutly nothing, it doesn't mix or change or washout the grease in any way, it evaporates or displaces and goes away. Sand, grit and others can hurt the life over time but on the back of a car is no less or more chance than general riding around town. Now... depending on amount of exposed steel screws/frame etc and things, you may get some surface rust but that is not a bearing wear issue. The car you are putting your bike on has the same exact style radial bearings on it's wheels as the bearings on your bike.