library(ggplot2)
library(dplyr)

# hwy : highway miles per gallon
#  1mile == 1.60934km
#  1gallon == 3.78541L
#  ∴ mile / gallon = 1.60934 / 3.78541 km / L

mpg2kpl <- function(x) x * 1.60934 / 3.78541

mpg %>%
  ggplot(aes(displ, hwy)) +
  geom_point() +
  scale_y_continuous(sec.axis = sec_axis(~ mpg2kpl(.), name = "km/L")) +
  labs(y = "mile/gal") +
  theme_classic()