In Jalica, domestically produced oil is more expensive than imported oil. Jalicans who get cars that are more fuel-efficient tend to do more driving than before. The Jalican government did not intend the new fuel-efficiency requirement to be a pollution-reduction measure. Some pollution-control devices mandated in Jalica make cars less fuel-efficient than they would be without those devices.
In order to reduce dependence on imported oil, the government of Jalica has imposed minimum fuel-efficiency requirements on all new cars, beginning this year. The more fuel-efficient a car, the less pollution it produces per mile driven. As Jalicans replace their old cars with cars that meet the new requirements, annual pollution from car traffic is likely to decrease in Jalica.
Which of the following, if true, most seriously weakens the argument?