The Jaguar XK120 began life as a one-off testbed for a new engine, but the reaction at its 1948 debut convinced Jaguar to build it — and the resulting car became the fastest production car in the world at the time. This 1952 example carries the original 3.4-liter inline-six good for 180 hp and a genuine 120-mph top speed. Jaguar built just over 12,000 XK120s between 1949 and 1954, with alloy-bodied cars now among the rarest of the breed.