NO! Please do not administer any over the counter medicines to your kitty, and especially not Xanax. Some human medicines can be given to cats, but the majority are toxic. Those that can be administered require specific dosing by your vet. A 'small' dose of xanax will cause alot of problems and may even be fatal.
I know that your kitty gets upset in the car, but it is dangerous to sedate pets for this purpose. In fact, there are so many known problems that airlines will not accept sedated pets as passengers or cargo any more. Pets that are sedated cannot regulate their own body temperature, and they can experience dangerous fluctuations in heart rate and blood pressure. And, your cats geriatric status (cats are geriatric at 8 years old) makes her a less than ideal candidate for sedation as these cats can sometimes experience insufficiency in kidney or liver function as part of the aging process. This is dangerous because the majority of the drugs used for sedation are metabolized with the help of the liver and excreted with the help of the kidneys.
My suggestion to you would be to place your kitty in a carrier and place a towel over the sides in it. Place her next to you in the car, and do not cover the front of the carrier. Make sure she can see you, and make sure that her carrier is well secured and won't go flying if you need to make a sudden stop. Keep the air temperature comfortable for her, and do not play any loud or jarring music. Talk to her and try to keep her as calm as possible, but please do not administer anything without talking to your vet. Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Good luck!