I mean it really depends. There are probably some circumstances where it's cheaper. But I think it's not a forgone conclusion. Renting costs money, and you'll need gas for it too. Motortrends estimates 1300 to 2000 a year in fuel for an Accord. And 400-1200 for electricity in a tesla model 3 (charging at home not a supercharger). I can easily imagine eating up the savings in one or two rental trips, and that's certainly less convenient.