Was this kitty feral before you started feeding her? I'm just wondering if she will be able to hunt to feed herself. It might help if you put up some signs in the area where you think she might be and describe her as orange and brown and maybe offer a reward? Even if you could get some reports from people if they've spotted her, that would help you narrow the search and hopefully get her back.
I guess my biggest fear is that she became injured while falling out of a moving car and is out there alone and may need medical help. I suppose it's possible that she is uninjured but that would take a miracle.
Is there a way you could place an ad in a local paper? Or put some signs up at some stores close to the location she went missing?
If she isn't feral and is familiar with sounds like dry cat food being shaken in a bag or a box, you could go to the area you think she is in and make these sounds when the traffic is quiet so she has a better chance of hearing you and vice versa. She'll be getting pretty hungry by now. Probably be best to walk the area rather than drive if you are going to continue looking for her as she is likely very scared of the car.
Please don't give up on her. You are the only one that knows she's out there. I'm praying for her.