My Other Car
My Other Car
[The back of a blue Mitsubishi with a spoiler is shown.] Bumper sticker: This IS my other car.

This comic refers to a popular form of bumper sticker that follows the template "my other car is a ____." Sometimes the blank is a fancy vehicle like a Porsche or a Ferrari ; sometimes it's related to the person's job (e.g. "My other car is a fire truck"); sometimes it's an even more expensive form of transportation like a " yacht " or " private jet ," or even something joking or in fiction (like a " TARDIS ").
The premise was to jokingly imply that someone driving in a less fancy vehicle was wealthier than they looked, as they could afford a fancy car (they simply chose to drive the clunker that day). The designer of the first stickers might even have intended them for serious use by wealthy drivers. The form of sticker ultimately became so well known that the phrase entered the pop-culture lexicon.
Due to their popularity, these stickers also have been parodied in various ways, like the one Randall has invented here. Randall's sticker is a more "honest" sticker that admits "this IS my other car;" in other words, this is the nicer of the two cars.
This sticker could probably be used on an expensive car to mirror the traditional sticker's use on a cheaper car. However, the car in the strip is a Mitsubishi, which is not a particularly expensive brand, though the presence of a spoiler indicates it may be one of the top-range models, or at least has had a little extra paid for some sports 'extras'. Thus it appears that Randall is using the sticker for contrasting purposes: while others would drive a modest car but joke that they have a really nice one at home, Randall's car is the one you see, and, as he noted in the title text, his other one is much worse than this one.
It's also possible that this is a play on meta-levels; by definition, the car that you're driving can't be your other car, as it's your car you're driving now. Your other car is the one sitting at home.
[The back of a blue Mitsubishi with a spoiler is shown.] Bumper sticker: This IS my other car.
