There's enough of a
debate in progress over
whether guys should open doors for girls, but opening a car door for a girl is
completely different. Unless, of course, you are going to go through the same car door, in which case the regular rules would apply. Even though
there are no rules.
In cars, however, everything is different because the guy opening the door for the girl is generally going to go around and get in through a different door (or he's going to get out through a different door first, but I don't know too many girls willing to sit there and wait for a guy to come open the car door for them before they get out). And this situation makes opening the car door for girls highly impractical.
First, in extreme temperatures, it's just that much longer she has to wait before he can get in and turn on the heater or the air conditioning. If it's raining, it keeps him in the rain longer. This is assuming that she isn't the one driving; if the girl is driving, then she probably has to open her own door anyway because she has the keys. If the car has been left running, or if the temperature within the car is comfortable, this isn't a factor. But it's still that much longer until he can get in the car, which means precious moments of separation while she sits alone inside the car and he walks around the outside. This separation can perhaps be countered by making silly (or sweet) faces at one another, but faces can be made over the top of the car or once inside, plus you can talk then, too.
Of course, if both get in the car at the same time, the girl loses a few moments of solitude in which to make adjustments to her wardrobe. Not a problem for the creative. If she's too embarrassed to pick a wedgie in male company, it's a simple matter of leaning forward and partially rising, pointing out some pseudo-interesting feature of the surroundings with one hand, and adjusting wayward undergarments with the other.
The only time when it is not only not impractical, but highly practical as well, for the guy to open the car door for the girl is when she is in a state of fancy dress. I went stag to my prom in a dress with a heavy cascade of flowing chiffon and satin skirts, and I really wished there had been someone to pick up my skirt and hand it to me. The most important point to remember on this as in any occasion when a guy feels it necessary to open and shut the door for a girl is: make sure you don't catch any part of her or her clothing in the door when you shut it. Hopefully any guy chivalrous enough to open the door for a girl is going to be careful about this anyway. Especially watch out for really long hair. I've slammed my own hair in the car door countless times.
And if you do really want to open the car door for a girl, go ahead. Just because it's often impractical doesn't mean it's never a good idea.