Splitting the file on Vehicle type has made the relationship between sales and fuel efficiency much more clear. There is a significant and fairly strong positive correlation between sales and fuel efficiency for automobiles. For trucks, the correlation is positive but not significantly different from 0. Reaching these conclusions has required some work and shown that correlation analysis using the Pearson correlation coefficient is not always straightforward. For comparison, see how you can avoid the difficulty of transforming variables by using nonparametric correlation measures.