Myrtle, watching Tom fill the car with gas, draws two wrong conclusions. What are they?

Myrtle does draw two erroneous conclusions: one implied and one direct.  First, Myrtle assumes that the big yellow car belongs to Tom.  Myrtle does not say this directly (and Nick doesn't tell us about it either); therefore, we have to imply that this is the case.   Of course, the reader knows the car belongs to Gatsby and that Tom is simply using the car to drive to town.  This is a very important error because eventually it leads directly to Myrtle's death.  Later, Myrtle sees the yellow car again.  Thinking it's Tom driving (like she saw earlier), she runs out without thinking.  Tom, of course, isn't driving the car.  Daisy is.  Daisy plows right over Myrtle, killing her. 

Second, Myrtle assumes that Jordan Baker (who just happens to be in the car) is Tom's wife.  We learn this error in judgement directly from Nick:

I realized that her eyes, wide with jealous terror, were fixed not on Tom, but on Jordan Baker, whom she took to be his wife. (Fitzgerald 125)

In my opinion, this erroneous conclusion isn't as important as the first in that it only indirectly leads to Myrtle's death.  Why?  Because anything that would put her into that kind of jealous rage would lead to very erratic actions, such as running out blindly in front of a speeding yellow car.