How long did they travel in the cars, and what did they eat?
I assume you are referring to the first time that Elie and his community were herded into wagons and taken to Auschwitz. We are not told precisely how long they were left in the cars, but we do know that they travelled for two days until they reached Kaschau before carrying on to Auschwitz. They ate the food that they had packed to take with them, but it is clear that they ate sparingly, trying to hoard and save for an uncertain future:
There was still some food left. But we never ate enough to satisfy our hunger. Our principle was to economise, to save for tomorrow. Tomorrow could be worse yet.
Note here how their attitude to food reflects an underlying uncertainty and despair about their future. In spite of their want they choose to save their food rather than eat it all now to hopefully give them sustenance in the uncertain days ahead.