https://data.bls.gov/cgi-bin/cpicalc.pl?cost1=9300&year1=197805&year2=201905 says that $9,300 in 5/1978 is CPI-inflation-adjusted to $36,924.89. So $42,000 is about a 14% price increase. But a 2019 BMW 320i is a much more capable car in almost every respect (other than fitting into a small parking space) than a 1978 BMW 320i.