Originally Posted by ISP_guy
They are usually called hub rings or centering rings. You don't have to use them...but they make installing the wheels a lot easier. When you put the wheel on the hub, the wheel studs will be centered in the holes and it makes it easier to thread the lugnuts on. If you buy wheels from a place like the Tire Rack, they'll include them with your purchase.
...a wheel with a larger bore size, do you have to get an adapter ring or something?
Believe it or not, one of the best (and cheapest) places to get hub rings is eBay.
Here are the hub rings I use on my aftermarket wheels. These are aluminum and cost $15 on eBay...
View my fuel log 2015 Mirage ES 1.2 manual: 51.3 mpg (US) ... 21.8 km/L ... 4.6 L/100 km ... 61.5 mpg (Imp)