I bought a bike, used, off of a crippled old man. And I want to make it a winter beater, so I'm looking for old knobby tires to drill screws into. But when looking into it, I can't wrap my head around what size to get.

The current tire on the bike says it's 700x38c. But when I measure, physically with tape, the wheel is 25 inches in diameter. But google says a 700 tire is supposed to be 27.5 inches.

What the fuck is going on?