1000 km Zeltweg

The 1000 km Zeltweg (originally known as the 500 km Zeltweg) was an endurance sports car event held near Zeltweg, Austria. Originally based at the Zeltweg Airfield, the race moved to the Österreichring and was lengthened to a 1000 km distance and there it continued to be a regular event in the World Sportscar Championship until 1976.

In 1963, Formula One held its first exhibition event at the Zeltweg Airfield, located in Styria. The airfield had been modified in 1958 to allow it to be used for motorsports, using the runway and taxiways for straights. Following the successful event, the Austrian Grand Prix joined the Formula One calendar for the 1964 season. However complaints from drivers about the poor surface led to the FIA abandoning the circuit before a 1965 event could be held. Left without a major event, the organizers turned to the World Sportscar Championship and offered a 500 km event to take place starting in 1966. This event proved more successful due to the ability of the sportscars to handle the bumpy surface better than a Formula One car.

In 1969, due to demands once again from racers for a better circuit, the Österreichring was built in the mountains less than a mile from Zeltweg Airfield. Once the new circuit was completed, the sports car event moved to its permanent home. Due to the freshness of the track, the organizers expanded the event to a 1000 km endurance. The smoothness of the new racing surface allowed for greater reliability and greater ease in achieving the longer distance. The high-speed nature of the layout also allowed for quick races, with some events running under five hours. However, with rule changes in the evolved World Championship of Makes in 1976, the event became limited to a maximum of six hours due to the decrease in overall speed of the competitors.

...
Wikipedia