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 major motivating factors 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, the same horn which mimicked the Road Runner cartoon character
Developed specifically for NASCAR racing, the Superbird, a modified Road Runner, was Plymouth's follow-on design to the Charger Daytona fielded by sister company Dodge in the previous season. The Charger 500 version that began the 1969 season was the first American car to be designed aerodynamically using a wind tunnel and computer analysis, and later was modified into the Daytona version with nose and tail. The Superbird's smoothed-out body and nosecone were further refined from that of the Daytona, and the street version's retractable headlights(made of fiberglass) added nineteen inches to the Road Runner's original length. The rear wing was mounted on tall vertical struts that put it into less disturbed air thus increasing the efficiency of the downdraft that it placed upon the car's rear axle. For nearly 30 years the mathematic formula used to determine the exact height of the enormous wing was thought to be a highly guarded Chrysler secret. However, in the 1990s a retired Chrysler project engineer admitted publicly that the height was determined in much simpler fashion: it was designed to provide clearance for the trunklid to open freely. The rear-facing fender scoops were thought to ventilate trapped air from the wheel wells in order to facilitate brake cooling, but in reality they were to hide cut outs in the hood to allow wheel clearance due to the lowered height of the car for NASCAR. Ground clearance was 7.2".
NASCAR's homologation requirement demanded that vehicles to be raced must be available to the general public and sold through dealerships in specific minimum numbers. For 1970, NASCAR raised the production requirement from 500 examples to one for every two manufacturer's dealers in the United States; in the case of Plymouth, that meant having to build 1,920 Superbirds. Due to increasing emissions regulations, combined with insurance hikes for high performance cars, 1970 was its only production year.
"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.
On the street, the nose cone and wing were very distinctive, but the aerodynamic improvements hardly made a difference there or on the drag strip. In fact, the 1970 Road Runner was actually quicker in the quarter mile and standard acceleration tests due to the increased weight of the Superbird's nose and wing. Only at speeds in excess of 90 mph (140 km/h) did the modifications show any benefit.
Chrysler memos of September 1969 show that the Sales Programming staff was preparing to handle 1,920 winged Plymouths for 1970, but published figures say as many as 2,783 were built. The current figure generally accepted is 1,935 SuperBirds built and shipped to United States dealers, with anywhere from 34 to 47 allegedly heading towards Canada. The engine option question is again a sticky one, although the most frequently seen numbers report 135 426ci. Hemi SuperBirds and 716 440ci. Six-Pack editions, with the remainder powered by 440ci. 4bbl. motors. It is believed that over 1,000 Plymouth SuperBirds exist today.
In Autumn 1968, Richard Petty left the Plymouth NASCAR Racing Team for Ford's. Charlie Grey, director of the Ford stock car program felt that hiring Petty would send the message that "money rules none". However, the Superbird was designed specifically to lure Petty back to Plymouth for the 1970 season. Petty did reasonably well against strong Ford opposition on the NASCAR tracks that year, winning eight races and placing well in many more. A recent tribute to Petty's Superbird was seen in the 2006 film Cars with Petty voicing "The King", a stock race car bearing a strong resemblance to a Superbird, and even carrying the same number and paint color.
NASCAR's rules implemented for the 1971 season limited the "aero-cars" to an engine displacement of no greater than 305 cu in (5.00 l) or they had to carry much more weight compared to their competitors. While they were still legal to race, the power-to-weight consequences that would come with the smaller engine or the increased weight rendered the cars uncompetitive. This was the start of a trend of rules slowing down NASCAR, because the races were exceeding the technology of tires and safety over 200 mph (320 km/h). Ford in response also designed the 1970 Torino King Cobra with a batmobile-like nose, but it was abandoned.
The Superbird's styling proved to be a little extreme for 1970 tastes (many customers preferred the regular Road Runner), and as a consequence, many of the 1,920 examples built sat unsold on the back lots of dealerships as late as 1972. Some were converted into 1970 Road Runners to move them off the sales lot. In recent years, however, the Superbird has become quite valuable. A Superbird can fetch anywhere from US$300,000 to US$2,000,000. On BarrettJackson, bids for original Superbirds crossed US$1,200,000. Some manufacturers produce Superbird conversion kits for 1970 Road Runners and Satellites. Kits are also available for non produced 1971 and 1972 bodies for the Superbird. The Superbird and the Dodge Charger Daytona were each built for one model year only (1970 and 1969 respectively).