1970's Plymouth Superbird existed for one reason only - to win at NASCAR, the US stock car racing series. At that time, though, NASCAR took the 'stock' in stock car racing reasonably seriously - vehicles to be raced had to be available to the general public and sold in sufficient numbers, a requirement known as homologation. In 1970, in fact, NASCAR raised the production requirement from 500 examples to one for each of that manufacturer's dealers in the United States; for Plymouth, that meant having to build 1,920 Superbirds.
The Superbird was basically a modified Plymouth Roadrunner, but it was realised that, while it was OK on the street to have the 'aerodynamics of a brick' typical of American cars of the period, something a little better would help at high racing speeds. So, following the lead of the previous year's Dodge Daytona, the Superbird sported an aerodynamic nosecone adding nineteen inches to the length and containing retractable headlights, a slightly smoothed body and, to counter a tendency to lightness at high speed, a rear wing mounted high on ludicrously tall tailfins. The reason for those fins was mostly to give clearance beneath them to lift the trunk deck lid, but it probably didn't hurt that it put the wing into less disturbed air.
All the cars used for racing were of course fitted with the 426 Hemi engine, but for the street two lesser engines were available, the 440 Super Commando with a single 4-barrel carburetor and the 440 Six Pack with three two-barrel carbs. Only 93 street cars were fitted with the 426 Hemi; 665 took the option of the 440 Six Pack, and everyone else took the cheap option of the 440 Super Commando.
On the street, the nosecone and wing made quite an impression, but the aerodynamic improvements hardly made a difference there or on the drag strip; in fact, that year's Roadrunner was if anything a slight touch quicker down the quarter mile. At 90mph or greater, though, things were quite different.
The Superbird did reasonably well against strong Ford opposition on the NASCAR tracks that year, winning eight races and placing well in many more. It didn't hurt, of course, that Richard Petty, one of the all-time great drivers, was behind the wheel of a Superbird that year.
NASCAR banned the Superbird and all the other aerodynamic speedsters in the 1971 season, concerned about dangerously high speeds and cars that didn't look all that 'stock', and thus no more Superbirds were produced.
While the Superbird was a little extreme for 1970 - many customers preferred the regular Roadrunner - now, they're highly valuable. A car in good condition can reach $50,000-$70,000 or more even with the more common 440 Super Commando, and examples with the 426 Hemi fitted at the factory (retrofitted doesn't count) and in near-perfect condition have changed hands for a quarter million dollars.