Check out this Streetfire video of Jackie Stewart as he tries out the six-wheeler 1976 Tyrrell P34. Designed by Derek Gardner, it was an attempt at working around aero rules to reduce frontal area. The smaller tires would have reduced front grip, however, so a second set was engineered into the design. Along with the Brabham BT46B 'Fancar' from 1978, the six-wheeled Tyrrell was the most radical F1 car ever.
It first ran in the Spanish Grand Prix in 1976, and proved to be very competitive, with its 3.0-liter V8 pumping out 475bhp @10,000 rpm. Both Jody Scheckter and Patrick Depailler were able to produce solid results with the car, even capturing the top two podium steps in the Swedish Grand Prix. Although Scheckter is still the only driver to have ever won a race in a six-wheeled car, he left the team at the end of the season, insisting that the six-wheeler was 'a piece of junk!'
For 1977, Scheckter was replaced by Ronnie Peterson, and the P34 was redesigned to provide better aerodynamics. The P34B was wider and heavier than before and although Peterson and Depailler again showed some promise, the P34B struggled. Most agree it was because of the lack of development on the tires, and the added weight of the front suspension. For 1978, the P34 was abandoned, ending a legendary chapter in F1 history.
[Sources: Streetfire, Wiki]