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This year, the racetrack at Daytona has been repaved. The track was always faster than other tracks NASCAR raced at and several cars in a "train" were faster than single cars or smaller trains. This year is different, however, because two car groups are by far the faster than any other groupings. When a third car joins a tandem, all three slow down noticeably. Why is this? Why wouldn't it still be faster to have more cars pushing?

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I'm waiting for someone who knows the fluid mechanics. It is true that much of the drag is accounted for by the low pressure area in the rear, which is caused by flow separation. What is happening immediately behind does affect this flow, so it is entirely possible that the lead car gets a benefit as well. If not, then the speed wouldn't be any higher than the lead car could do by itself. The following cars could still save on fuel, which might allow them to be more aggressive at another point in the event.

I think all the participants in the race have acepted a high degree of risk as part of the cost of participation.

In bicycling, the advantage is considered to be to the following riders only. A friendly group will rotate riders so that all can share in the drafting.

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How close drive that followers in Daytona? If the leader shall have some benefit, the distance should be less than the diameter of the vortices generated at the rear end of the leading car. (If the follower is close enough, I suppose, that these vortices are less or smaller) –  Georg Feb 20 '11 at 22:01

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