1971-73 Buick Riviera: Average miles per gallon: 5-8 MPG, Average cost of one gallon of gas: $0.44 (1971) - $0.65 (1973). Inspired by a Bill Mitchell concept, Jerry Hirschberg's 'Boattail' Riviera design was considered by many to be outrageous, even ugly upon its debut. Hmm, critics! Thirty-seven years later, it has proven one of the most beautiful and distinctive designs of the era. Already increasing in value, a mint-condition Riviera can fetch as much as $25,000. Pre-emission models with the 455 cid engine are particularly desirable, as are the even rarer 1971 330-horsepower GS cars. (General Motors)