The short-lived Plymouth Road Runner Superbird was a highly modified version of the Plymouth Road Runner with well-known graphics and horn. It was the factory's follow up stock car racing design for the 1970 season to the Dodge Charger Daytona of 1969, and incorporated many engineering changes and modifications (both minor and major) garnered from the Daytona's season in competition on the track. The car's primary rival was the Ford Torino Talladega, which in itself was a direct response to the Mopar aero car. It has also been speculated one motivating factor in the production of the car was to lure Richard Petty back to Plymouth. Both of the Mopar aero cars famously featured a protruding, aerodynamic nosecone, a high-mounted rear wing and, in the case of the Superbird, a horn which mimicked the Road Runner cartoon character.
"Superbird" decals were placed on the outside edges of the spoiler vertical struts featuring a picture of the Road Runner cartoon character holding a racing helmet. A smaller version of the decal appears on the driver side headlight door. Superbirds had three engine options: the 426 Hemi V8 engine, the 440 Super Commando with a single 4-barrel carburetor, or the 440 Super Commando Six Barrel with three two-barrel carburetors. Only 135 models were fitted with the 426 Hemi. As the 440 was less expensive to produce, the "Street" version of the 426 Hemi engine used in competition was homologated by producing the minimum number required.
Source: Wikipedia (original)