With the strong sales in March, 2019 is averaging 16.9 million (average of seasonally adjusted rate), just behind sales last year (17.0 million average through Q1).
Click on graph for larger image.
This graph shows annual light vehicle sales since 1976. Source: BEA.
Sales for 2018 were the fourth best ever, and the 2019 sales rate is lower than in 2018.
The second graph shows light vehicle sales since the BEA started keeping data in 1967.
A small decline in sales this year isn't a concern - I think sales will move mostly sideways at near record levels.
This means the economic boost from increasing auto sales is over (from the bottom in 2009, auto sales boosted growth every year through 2016).