At what points should I start and stop the timer for career mode? I would say the timer should start when picking the car for the first year and the stop when you reach the finish line for the last stage.

I don't mind when it starts, I think on pressing A on your car is maybe slightly better, but I don't think it makes much difference.

As for ending, I think we should include the last "cutscenes" where you see your win% etc as part of the timer, as it's a different length for certain years, so you're kind of cutting off the end of the last years you do.

I haven't done the full research on this, but regardless of group, isn't the amount of races the same each year? What I mean by this is that no matter when we stop the timer we skip the same amount of time at the end anyways. The time it takes between you passing the finishing line and you having seen the "cutscenes" should be exactly the same length of time irregardless of which rallies you've been racing.

To me it seems more exact in to stop when finishing and not the guesswork of waiting for the cutscenes.

Yes but you might do them in a different order. Though now I think about it I'm not actually sure if that makes it take any longer. I guess it doesn't really matter, as long as you could in theory retime it down to the frame, just in case, it's specific enough.