<P> The oil crisis in the early 1970s led organizers to adopt a fuel economy formula known as Group C that limited the amount of fuel each car was allowed . Although it was later abandoned, fuel economy remains important as new fuel sources reduce time spent during pit stops . Such technological innovations have had a trickle - down effect and can be incorporated into consumer cars . This has also led to faster and more exotic supercars as manufacturers seek to develop faster road cars in order to develop them into even faster GT cars . </P> <P> Additionally, in recent years hybrid systems (flywheel, super-capacitor, battery coupled with both gasoline and diesel) have been championed in the LMP category as rules have been changed to their benefit and to further push efficiency . </P> <P> The race is held in June, leading at times to very hot conditions for drivers, particularly in closed vehicles with poor ventilation; rain is commonly seen . The race begins in mid-afternoon and finishes the following day at the same hour the race started the previous day . Over the 24 hours, modern competitors often cover distances well over 5,000 km (3,110 mi). The record is 2010's 5,410 km (3,360 mi), six times the length of the Indianapolis 500, or approximately 18 times longer than a Formula One Grand Prix . Drivers and racing teams strive for speed and avoiding mechanical damage, as well as managing the cars' consumables, primarily fuel, tyres, and braking materials . It also tests endurance, with drivers frequently racing for over two hours before a relief driver can take over during a pit stop while they eat and rest . Current regulations mandate that three drivers share each competing vehicle . </P> <P> Competing teams race in groups called "classes", or cars of similar specification, while also competing simultaneously for outright placing amongst all classes . Originally, the race showcased cars as they were sold to the general public, then called "Sports Cars", in contrast with the specialised racing cars used in Grand Prix motor racing . Over time, the competing vehicles evolved away from their publicly available road car roots, and today the race is made of two overall classes: prototypes, and Grand Touring cars (similar to sports cars sold to the public). These are further broken down into 2 sub-classes each, constructors' prototypes, privateer prototypes and 2 subclasses of GT cars . </P>

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