Okay, ignore the high wind stuff, that's just their get out of jail card. If a tent is installed correctly, ideally in a sheltered area, it will be fine. I've owned five car tents and never had a problem and never had to install storm guy ropes. The current tent sits on the driveway next to a high hedge that runs along full tent length...UK prevailing wind is mainly from the west so trick is to place front or rear facing west. The tent never moves, even in the strongest of storms. I have a neighbour with same tent sat in corner of his rear field, erected in similar east-west direction, it never moves.
A 12ft length may be too small and the important measurement is the front access height when driving into the tent.