From what I heard they were losing on both tires but worse on the car tire. In 2012 Jason was digging trenches 6 inches deap and fish tailing all over the salt (which was like riding on loose sand) It was just worse on the car tire but neither were something to brag about. I believe the salt flats is the only place that allows car tires. I know the 1 mile and 1.5 asphault tracks do not. I also heard besides the salt flats the Texas mile was the other course that the rules were more stringent.
Though I am very much pro DS on the street, I have to agree, it wouldn't be my choice for the salt. Not to mention the much heavier reciprocating weight.