Gatsby's car is the one that hits Myrtle. Daisy is driving it, and after she runs Myrtle over, she keeps on going.

Myrtle, however, believes the car belongs to Tom. This is because Tom is driving it when he stops to get gas at the station on the way to New York City. Tom taunts George Wilson by pretending it is his own and asking him if he wants to buy it.

Myrtle thinks Tom is driving the car when it is heading back home, as would make sense if it were his, so she runs out to try to hail him down. She wants Tom to save her from George, who has found out that she is having an affair and, therefore, plans to sell the garage and move them out of the area.

George also believes Tom was driving the car and arrives at his home planning to kill him in revenge for running over and killing Myrtle. Tom, however, callously directs him to Gatsby, saying it was his car and suggesting that he was driving it.

Myrtle was killed by Jay Gatsby's car.  She thought that her lover, Tom, was driving the car.  

Earlier, Tom and Gatsby exchanged cars while in New York.  Myrtle saw Tom driving the car, and assumed it was his car.  Then when Daisy and Gatsby were coming back home, Gatsby and Daisy were back in his car.  Myrtle ran out into the street in attempt to flag down who she thought was Tom.  Daisy happened to be driving Gatsby's car at this point, and was so upset by earlier events that she was not able to correctly handle the vehicle.  Sadly, Daisy struck and killed Myrtle.  

Jay Gatsby is so in love with Daisy that he is willing to take the blame and the consequences of the accident.  He plans on telling everybody that he was driving, not Daisy.  

‘Was Daisy driving?’

‘Yes,’ he said after a moment, ‘but of course I’ll say I was.

Myrtle saw Tom driving Gatsby's car while they were in New York. Myrtle is trying to run away from her husband, who she thinks is going to try to take her away from all of her friends, because he believes she is having an affair, which is true. She runs out and sees Gatsby's car, and thinking that it is Tom driving, she tries to get the car to stop. Sadly, it is Daisy, who is driving the car, and she is so upset, that she strikes and kills Myrtle. 

When Gatsby realizes what has happened, he wants to say that he was driving the car, not Daisy. This is a pivotal look into the character of Gatsby. His love for Daisy is so great, that he is willing to take the blame for Myrtle's death so Daisy won't have to face any consequences for her actions. It is ironic that Daisy kills her husband's mistresses, and doesn't even know what she was to Tom, and that it was done with Gatsby's car, whom Daisy has also been involved with.

This scene sets up the actions for many people. In the end, it is because of this accident and Gatsby's car, that will eventually lead to the tragic outcome for Gatsby. 

Myrtle runs out to hail a car she believes belongs to Tom. Myrtle had seen Tom driving this car earlier when he and Gatsby swap vehicles for the drive to New York.

She rushes to the car because her husband is planning to take her away. He suspects that she has been having an affair. 

The car she tries to hail and stop is actually Gatsby's car, but Gatsby is not driving. Daisy is driving. Upset after the explosive and emotional confrontation in New York, Daisy is driving in an agitated state. 

Gatsby is willing to take the blame and claim that he was driving, but the fact is it was Daisy who hit Myrtle and Myrtle believed she was running out to Tom's car.