For some people, a car is a utility that gets them from one destination to the next. For others, a car is an expression and extension of their personality. That’s how we get political bumper stickers, Garfield window decorations, and custom paint jobs that may or may not involve flames.
One area where people can get extra fancy is in their wheels, or rims. But rims aren’t exactly wheels, and hubcaps are neither. So what’s the difference?
In the strictest sense, a wheel is what you think it is—a circular method of locomotion. A slang term for a car wheel is a rim, but technically, the rim actually refers to the outer edge of the wheel that helps keep the tire in place. It’s similar to the rim of a drinking glass. People typically use wheel to describe the wheel, rim, and rubber tire.
So why do people use rim when they mean wheel? It could be because flashier, larger wheels have more prominent rims, but it’s hard to know for sure. (Green’s Dictionary of Slang dates rim to mean wheel to 2000, so it might be a relatively recent development. Green’s also helpfully informs us that rim slide is prison slang for flatulence. Do with that what you will.)
A hubcap is the plastic or metal cover that goes over the center part of the wheel, or wheel hub, which is how the wheel is attached to the vehicle. Typically, a hubcap guards against rain and other elements that can lead to corrosion. Not all wheels require hubcaps, as modern cars have wheels made of aluminum or metal alloys that don’t wear or corrode like steel.
Cars can also sport wheel covers, which cover the entire diameter of the wheel, but people still usually call this a hubcap. Why? Likely because hubcap has become a catch-all term for anything covering the wheel. Strictly speaking, though, a hubcap doesn't need to extend to the rim: It just needs to cap the wheel hub.
You can also think of it this way. Rims are necessary in order to secure the tire to the wheel; hubcaps are optional. So is a car, depending on the walkability of your city, but that’s another story.