Calvin, while running away from home after they break the car.
Calvin's parents have a car that is involved in a few of the story arcs. Calvin sometimes wrecks the car, but never in a story arc is it totaled or even damaged, and this mostly appears in throwaway jokes or single strips. One time, when he and Hobbes were clearing out the garage so they could have G.R.O.S.S. meetings in it, he accidentally pushed the car into the ravine across the street from his house. This is the paramount story arc involving the car. Another time, Calvin tried to sell the car to get some money (single strip).  Yet another time, Calvin wrecked the car so badly that he imagined that if his dad] could find it, he and Hobbes could pretend it might be fixed(again, this was only a single strip), after which Hobbes gives Calvin a book (likely a Spanish dictionary, as Calvin is reading the book and saying "Que pasa, señorita? I am el fugitivo! Which means "What happens, miss? I am the fugitive! )While Hobbes Is holding a suitcase and says "You keep the book, I'll call the bus station."This suggests that the damage to the car is fairly severe. Calvin also tells his parents to do daredevil stunts, like pulling him behind with a rope  or jumping off of a ramp made out of snow, "[leaving] a patch of molten rubber in the garage." 
- In one Rosalyn strip, when Calvin and Hobbes noticed their parents leaving, Hobbes suggested the idea of jumping into the other car and learning how to drive. It has never been depicted of there being another car in the garage.
- The car appears to be a Ford Escort hatchback.