In 1955, the "Two-point-four" or 2.4-litre saloon (named by enthusiasts 2.4 Mark 1) was the first monocoque (unitary) car from Jaguar. Its 2.4-litre short-stroke version of the XK engine provided performance. In 1957, the 3.4-litre version with disk brakes, wire wheels and other options was introduced, with a top speed of . In October 1959, an extensively revised version of the car with wider windows and 2.4, 3.4, and 3.8-litre engine options became the Mark 2. The 3.8 Mark 2 was popular with British police forces for its small size and performance.