The Brabham BT55 was a Formula One racing car designed by Gordon Murray and David North for the Brabham team owned by Bernie Ecclestone. It used a BMW four-cylinder turbocharged engine tilted over on its side to allow a clear supply of air to the rear wing. The car competed during the 1986 Formula One season. It was not successful and its introduction coincided with the end of Brabham's time as a competitive team. Murray's next car (which he helped Steve Nichols design), McLaren's MP4/4, is usually claimed to be based on the same principles and won 15 of 16 races in 1988.

By 1985, Brabham had reached the limit of aerodynamic development of their BT52-BT53-BT54 series of cars. The 1985 car won only one race when Piquet won in France, and Murray decided that a radical approach was needed. With ground effect having been banned a few years earlier, the rear wing of a Formula One car once again created much of its downforce, but its effectiveness is reduced by the bodywork in front of it disturbing the flow of air. Brabham's tall, relatively heavy straight-four BMW M12 engine was particularly difficult to package to allow a good flow of air to the rear wing. Designers in the 1950s had addressed the same problem of reducing the cars cross sectional area by tilting the engines around a vertical or longitudinal axis by a small amount. Examples include the 1950s Kurtis-Kraft and Epperly Championship Cars and Colin Chapman's Formula One Lotus 16.

Both the Brabham team and their gearbox supplier Weismann lay claim to the idea of doing this with the tall BMW engine in order to create a car with very low bodywork that would allow a large supply of air to reach the rear wing undisturbed and create more downforce without harming the straight line potential with high drag. The driver was placed in a lying down position (approximately 30° according to Murray), as had been common in the 1960s, but had become rare by the 1980s. BMW designed a special version of their four-cylinder turbocharged engine with the engine block tilted almost horizontally (18° from horizontal).

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