Completely depends on the car. Wife has a 2006 Mariner, it has 19mm wheel nuts. My '91 Explorer is 5/8" or 9/16" or so (I don't remember). I also have a BMW tha

Completely depends on the car. Wife has a 2006 Mariner, it has 19mm wheel nuts. My '91 Explorer is 5/8" or 9/16" or so (I don't remember). I also have a BMW that's 100% metric of course. And a '68 mustang that's 100% SAE. The explorer is metric in a lot of places. The engine is SAE since it's based on the 302 which was originally designed in the early '60s. Pretty much anything that was designed from the mid 80s or later will be 100% metric hardware unless it needs to work with something that isn't. And even then it'll will sometimes be metric until it hits the non-metric piece. For example, the Tremec T-5 transmission is metric but mates up to the Ford 302 which is SAE. So basically, it's usually metric except when it's not.