Most of the time(indeed, pretty much all of the time), it's the fastest car that wins both the driver's and constructor's championships. There are a few exceptions, one of the most famous is 1986 and Alain Prost's McLaren TAG betting the superior Williams Honda to the driver's championship. I am not saying that Prost didn't deserve it(he's one of the all time great drivers after all), but it certainly helped that Williams had two drivers taking points off one another compared to Prost's situation at McLaren.
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