In automobile design, an MR or mid-engine, Rear wheel drive layout is one in which the rear wheels are driven by an engine placed just in front of them, behind the passenger compartment. In contrast to the rear-engined RR layout, the center of gravity of the engine is in front of the rear axle. This layout is typically chosen for its low polar inertia and relatively favorable weight distribution (the heaviest component is near the center of the car, making the main component of its moment of inertia relatively low). The layout does suffer from a tendency toward being heavier in the rear than the front, which is not ideal for handling. However, it is generally felt that the lower polar inertia more than makes up for this. The mid-engined layout also uses up central space, making it impractical for any but two-seater sports cars.
In modern racing cars, MR is the usual configuration and is usually synonymous with "rear engine". Due to its favorable weight dynamics, this layout is heavily employed in Formula racing cars (such as F1s).