Automakers are just starting to figure out how to transform used car owners into subscribers. Today, most of them reach used-vehicle owners either when someone buys a used vehicle through a certified dealer (about one-third do in the US) or when that new owner seeks out its app. (Automakers also reach out to owners in the case of recalls, through highly regulated contact with local motor vehicle departments.)

General Motors spokesperson Anna Yu declined to share specific numbers on subscribers who drive used cars, but she says that “second…