Jefferson family's car

The Jefferson Family's Car is a luxury car owned by Rowley's father, it was shown in The Third Wheel and Dog Days(film)

The Third Wheel
In its only appearance in The Third Wheel, when Greg heard that Abigail Brown was coming with him and Rowley for the Valentine's Day Dance, Greg said that he didn't want his minivan as the ride because it was dirty and Greg thought Susan would embarrass him, so he choose Rowley's car.

When going inside, Greg found that Rowley still uses a backless booster seat, however it was covered up as Greg said Abigail thought Rowley was trying to be funny.The passenger seat had papers so Greg had to sit way in the back.

In the back, there was the vacuum cleaner in the back which Robert picked up from Bob's Vacuum Cleaner Repairs, the vacuum was dirty and hence dirtied Greg's suit jacket, making him leave it when going to Spriggo's.

When arriving at Spriggo's when finding it was mandatory for men to wear jackets, they give Greg a substitute one which was very dirty, Greg tried to clean it but it was of no use, so they left.

Dog Days (film)
It appeared as a 2001 Volvo V70 XC in the film.

Trivia

 * Greg mentions that the car has a seat in the way back which faces the rear window, much like the Heffleys' old station wagon.
 * The car has no visible gas cap or tailpipe. This hints that the car may be an Tesla Model S electric vehicle, as the Tesla also has the rear seat mentioned earlier, however the headlight pattern means that the car looks more like an BMW 5 Series and no gas caps or tailpipes are actually drawn on any other car in the books, so the model is either a fictitious fusion of several or is unknown.
 * The car has FM Radio, as seen in the 3rd Wheel when Mr. Jefferson is listening to a Dow Jones report