There is no metric for "inflation for cars". What the BLS data is saying is that the nominal price/quality for cars has stayed the same. So in 1990 you could get a mustang for $20k, and if instead you stuffed 20k in you mattress for 30 years, you could now buy a base model Kia, which when all the positives and negatives are added up delivers similar value to the features of a 1990 mustang. A 2021 mustang costs a lot more, but it's obviously in a different league, hence the adjustment.

Meanwhile, that $20k buys at loss less food, healthcare, education, house, etc which is why "today's dollars" are worth less and why it's fair to say cars got cheaper in inflation adjusted terms.