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 near-ideal weight distribution. The mid-engined layout does, however, use up a lot of space, making it impractical for any but two-seater sports cars. Though it is sometimes advantageous to have more weight in the rear, during cornering it makes entry a lot more difficult as the front tires have less traction, resulting in a slight understeer, and at the exit of the turn, the heavy rear tends to cause oversteer.
Early cars using the MR layout included the 1930s Auto Union racers, Cooper racers, Zündapp Janus (1957), De Tomaso Vallelunga (1965), Lotus Europa (1966), Lamborghini Miura (1966), Ford GT40 (1966), Ferrari Dino 206/246 (1968), and Porsche 914 (1969). Its space-inefficiency means that it is still only used in sports cars and supercars such as the Toyota MR2, the Enzo Ferrari, Honda NSX, Lotus Elise, MGF, and Pagani Zonda. The Zündapp Janus is perhaps unique, in that it is literally mid-engined and nearly symmetrical with passengers on both ends of the engine.
In modern racing cars, MR is the usual configuration and is usually synonymous with "rear engine". Due to its near-ideal weight distribution, this layout is heavily employed in Formula racing cars (such as F1s).
This configuration was also common in very small engined 1950s microcars, in which the engines didn't take up much space.