I've noticed the same thing about the challenges. I think they did that last season. The Avalanche made good TV, so they gave them the win when they were clearly slower than the others a couple times.
That's my opinion, anyway. I'm wondering if it happened with the Lambo too, cuz they didn't appear to be quite as fast as either the Cuda or the NSX, but it wasn't nearly as blatant as with the Avalanche.
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