In Chapter Seven of The Great Gatsby, because the Buchanans, Jordan Baker, Nick Carraway, and Jay Gatsby decide to go to New York City on a "broiling" day, Myrtle Wilson makes two false assumptions.
First of all, there is a tension in the air since Daisy and Jay Gatsby have begun their affair and Daisy acts unconscionably by kissing Jay earlier in her home. Perhaps because he suspects that something is going on and he thinks Gatsby is not who he claims to be, the supercilious Tom Buchanan declines Gatsby's offer that they all go in his car. Instead, he tells Gatsby to take his coupé and he will drive Gatsby's automobile. Since Gatsby does not really like this idea, he suggests that the car may not have enough gas; however, Tom will not be deterred. He says that he can buy some gas.
Later, when Tom Buchanan pulls up at Wilson's garage in Gatsby's car, Myrtle Wilson peers out an upstairs window. She may overhear Tom when he says to George, "How do you like this one?" and George replies, "It's a nice yellow one." As she listens, Mrytle is so engrossed that she has no consciousness of being observed and one emotion after another passes onto her face
...like objects into a slowly developing picture....Her eyes, wide with jealous terror, were fixed not on Tom Buchanan, but on Jordan Baker, whom she took to be his wife. (This is the first of Myrtle's false assumptions.)
Naturally, since Myrtle assumes that Jordan is Daisy Buchanan she also assumes that Tom is driving his own car. (This is her second assumption.)
After talking with Wilson, Tom pulls away hurriedly, both because he is nervous after Myrtle's husband confides that he has learned something about his wife, and also because he is worried about what Daisy and Gatsby are doing together in the coupé that he has seen speed past.
Myrtle's assumptions may well seem plausible to her. For, she probably mistakenly assumes that Tom does not want his wife to see Myrtle while he is there at the garage.
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.