Hi,
Both my boys hated car setas and travelling in the car, on EVERY occasion and i can't remember when it wore off but it was months. My daughter on the other hand was a car- sleeping baby.
Like you i would use the car as little as possible as it was so distressful for both me, baby and other kids. I took bus or walked as much as possible but was not always ideal as lived in middle of nowhere.
I tried everthing, even borrowed a different car seat off a friend and bought a second hand britax first class, but it was no use. My boys were never diagnosed with reflux but were very demanding babies in general, the little one cried alot but doctors found nothing wrong.
Travelling in the car really sent their screaming into a whole new level and it was very stressful, we had to cancel any plans to travel any long distances (more than 30 mins) as the babies were very very distressed.
We never found the reason why.
sorry this is no help but i couldn't find any answers to it, maybe it was reflux or car sickness but my little boy cried in the car back from hospital and we went from there. He is 3 now and falls asleep in the car.