When I was young, our family transported 2 cars on QE2, both from Cherbourg to New York, in 1975 and again in 1977. I still have the August 15 1977 "car passage ticket" in my QE2 artifact collection. Both Cherbourg and New York were drive on - drive off for cars.
We had to arrive at the pier in Cherbourg by 10:00 AM the day of sailing. After check in, they did a "steam cleaning" which entailed picking up the car by crane, and having the underside washed via a fire hose; I was never entirely sure if it was salt or fresh water! My father had a certificate delivered to his cabin on each crossing that the car had been "steam cleaned" as it was required for entry into the US. On both voyages, there were well over 30 cars loaded at Cherbourg, I have pictures of our car(s) parked and waiting in queue for each voyage. QE2 did quite well as a car ferry in those days.
In 1975, the fellow operating the crane was accidentally sprayed with water from the hose by the car cleaner below. He yelled obsenities at the man below, and proceeded to rapidly move a huge lime green Plymouth Fury out of range of the man below. Needless to say, he overdid the move, and the Fury proceeded to crash head on into the corner of an adjacent crane, similar to driving head on into the corner of a building. The loud crash occured, the green antifreeze and some other fluids leaked from the car onto the ground. The car was quite badly damaged. A minute later, a large, loud enraged New Yorker who owned the Fury stormed out of the terminal lounge, and was yelling obscenties at the French stating he was going to "kick their a$$es". He was ultimately prevented from reaching the workers, but it was all quite dramatic. I am not sure if the green Fury made that voyage, it was just put down and pushed into a corner of the car waiting area as it was not running. After we boarded, I was focused on exploring the ship as it was my first time aboard QE2. Quite the adventure at Cherbourg that day!