To cite package 'mpg' in publications use:

Thomas J. Leeper (2014). mpg: FuelEconomy.gov Data. R package version 0.1.

Corresponding BibTeX entry:

  @Manual{,
    title = {mpg: FuelEconomy.gov Data},
    author = {Thomas J. Leeper},
    year = {2014},
    note = {R package version 0.1},
  }