There is no elegant way to do this YET, at least for wheels, because the brake disc ideally needs to be part of the wheel model (to spin), but discs are often unique per car (offset etc) and also tied to the brake caliper.
If Ruud could allow us to define multiple parts for the wheel (ie, disc/wheel and tyre separately), then we could happily do this 'sharing' thing a lot better.
We still need some good standards for polygon levels though.
In theory, it'd be nice to build to strict standards on tyre sizes and edge count, so you could swap tyres easily...
Lots of potential, but with it all tied in to one rotating item definition for now, the solutions are not so elegant for parts swapping around wheels etc unfortunately