michael769 wrote:Modern alloys mean that the rims can be much lighter than the tyres - hence you want bigger rims and smaller tyres.
I'm wondering why you think this - have you weighed any wheels and tyres to check?
Have you weighed racing wheels?
On a track racing car you don't really need your rims to survive 100,000+ miles of potholed roads so you have options that are not available to the manufacturers of rims intended for road use.
The composites and alloys used in racing wheels can have densities of 80% of that of tyre rubber (and I've heard of composites in the pipeline that can get down to 60%). Furthermore there are more opportunities on a wheel to reduce the weight further (thinner metal, narrower spokes etc) that you cannot do to tyres.
That is very different to what you will experience on a road going car - the most common alloys used are around twice a dense as tyre rubber, and the aforementioned need for them to survive on the public roads means you need considerably more material on them.
Road car rims are 80-90% cosmetic - they just need to look sporty - not be sporty.