inca was bad for being car sick, it was only after her first trip to the beach which was a 2 hour drive that she stopped.
how long into the car journy is she sick, does she cry at all before hand, where does she travel(frount/back seat or boot/trunk)
have you tried not feeding her.
if you have a drive way, take her out and sit in the car give her treats if you want, then take her back in. do that a couple of times then do the same but this time turn the engine on and sit for a bit, again repeat. them once she is ok with that, start moving the car, reverse the car out the drive then back in, and repeat a couple of times.
If it is on the motorway she is fine but even driving carefully around town she is more likely than not to be ill. I have he in the front with me, either on the passenger seat or footwell. She does not stress or cry.
I will speak to the vet and see if there is a remedy.