My Macy got car sick right form the start. The day that we rescued her she got sick on the ride home (about 30 minutes). I got in the habit of letting her lick my hand as when she stopped licking we knew she was going to get sick.
While shopping at Pet Smart, I found a seat cover that attaches to the Headrests on the Back seat and the Headrests on the Front seats. Ever since I did this she has only got sick once. Also, she used to only lay down for short periods and never seemed to be able to get comfortable but now she is laying down within a few minutes of travel. She will get up when I stop but as soon as I start moving down she goes. It seems as though she feels safer as she can not fall off of the seat. If I remember correctly the seat cover was right around $50.00, so it might be worth a try.