First of all, I would not encourage a chihuahua to ride with her head out the window.... It is far too easy for a little dog to decide to jump out the window.
Usually car sickness is a reaction to stress - if she's not used to riding in the car, or has had bad experiences after riding in the car, she could be stressed. I would suggest trying to figure out how long she rides before she starts feeling sick, and then keep your trips shorter than that for a while. So say you notice that she starts feeling sick (and I don't mean vomiting, I mean that you can see that she doesn't feel well) when she rides for 10 minutes.... so then take her for a few 5 minute car rides. Even if you just drive around the neighborhood and back home, you'll need to get her some good experiences in the car. Then after about 5 short trips, start taking slightly longer trips.... still not 10 minutes, maybe 8 minutes or whatever. Do about 3-5 of those trips, then try the longer trip and see how she does.
Many dogs also do better if they ride confined in a crate so that they can't see out windows. If she can't ride in a crate at least confine her so that she can't move around too much - you can get a dog carseat or seatbelt. Some people also give their dogs ginger dog treats before the car ride, the ginger helps settle the stomach.