Sorry totally disagree with the above answer, it is often the fact that they can see out that makes them sick. It is a combination of the motion and seeing things wizzing past that upsets their balance.
Try putting her in a carrier or a cage covered with a sheet, so she cannot see anything moving past. Put her on the floor of the car, or at least in the back if its an estate or a hatchback, or even better is to let her travel in the passenger footwell, nearest the gear stick.
Let her spend some time sitting in the car without it moving
and when you do take her out in it, make sure its somewhere nice like a short trip to the park for a walk.
She will grow out of it, but you can make it easier for her.
Make sure you don't feed her for a few hours before, and make sure she has some water available in a non spill bowl. Use newspaper or towels so you can easily clear up after her if she is sick without making a big thing of it.