The fourth generation of the Ford Thunderbird is a large personal luxury car produced by Ford for the 1964 to 1966 model years. This generation of the Thunderbird was restyled in favor of a more squared-off, "formal" look. The Thunderbird's sporty image had by that time become only that: the standard 390-cubic-inch 300 bhp (224 kW) V8 engine needed nearly 11 seconds to push the heavy T-bird to 60 mph (96 km/h). The softly sprung suspension allowed considerable body lean, wallow, and float on curves and bumps. Contemporary testers felt that the Buick Riviera and Pontiac Grand Prix were substantially more roadworthy cars, but the Thunderbird retained its leading market share.