AMC 10 2017b Problem 12

Elmer's new car gives $50\%$ percent better fuel efficiency, measured in kilometers per liter, than his old car. However, his new car uses diesel fuel, which is $20\%$ more expensive per liter than the gasoline his old car used. By what percent will Elmer save money if he uses his new car instead of his old car for a long trip? $\textbf{(A) } 20\% \textbf{(B) } 26\tfrac23\% \textbf{(C) } 27\tfrac79\% \textbf{(D) } 33\tfrac13\% \textbf{(E) } 66\tfrac23\%$