When Myrtle watches her husband fill Tom's car with gas, what two conclusions does she draw?

When Myrtle sees her husband, George, filling the car Tom Buchanan is driving with gas, she assumes that the woman in the front seat, Jordan Baker, is really Tom's wife, Daisy.  Nick turns to see her staring out of the garage's second story window, her eyes "wide with jealous terror," and he realizes her mistake.

We only realize Myrtle's second assumption later, when Nick, Jordan, Tom, Gatsby, and Daisy are on their way back from...

When Myrtle sees her husband, George, filling the car Tom Buchanan is driving with gas, she assumes that the woman in the front seat, Jordan Baker, is really Tom's wife, Daisy.  Nick turns to see her staring out of the garage's second story window, her eyes "wide with jealous terror," and he realizes her mistake.

We only realize Myrtle's second assumption later, when Nick, Jordan, Tom, Gatsby, and Daisy are on their way back from New York City and going through the valley of ashes en route to East Egg. George had seen Myrtle rush out to that same yellow car, the car that Tom had been driving earlier when he stopped for gas, when she saw it coming down the road. He tells his neighbor that "She ran out to speak to [the man who was driving it] and he wouldn't stop." Now, George obviously knows that it wasn't Tom driving that car because he was driving his own blue coupe that stopped at the time; however, Myrtle didn't know that (she was already dead by the time Tom stopped) and so she must have assumed that it was one of Tom's cars and that it would be him driving it home. That yellow car, of course, belongs to Gatsby, and so George assumes that this is the man with whom she'd been having an affair.