Correlations

Correlation matrix for Fuel efficieny and Log-transformed sales, split by Vehicle type, using Spearman's rho as the measure of correlation. — Figure 1. Correlation matrix

Spearman's rho is reported separately for automobiles and trucks. As with Pearson's correlation coefficient, the association between Log-transformed sales and Fuel efficiency is fairly strong. However, Spearman's rho reports the same correlation for the untransformed sales! This is because rho is based on rank orders, which are unchanged by log transformation. Moreover, outliers have less of an effect on Spearman's rho, so it's possible to save some time and effort by using it as a measure of association.