Bigger wheels have advantages in braking (bigger tread patch) and cornering (sidewalls do not deform as much).
As one automotive engineer told me, they are using bigger wheels nowadays because they can. Car bodies today have much greater torsional rigidity than a decade ago, so they can tolerate stiffer suspension and bigger wheels.
I believe that having a low profile tyre combined with bigger rim, but keeping the total wheel diameter the same, will actually increase MPG.
Although it is implicit in all of the above posts, no one ever actually said this explicitly:
GuanoLad, you’ve probably gathered by now that the word “rim” is usually used to refer to the thing that you have called a hub (i.e., the large metal thing that the rubber tire sits on.) That’s where the early-post confusion came from.
(For the record, I’ve never thought “rim” was a good name for this item. I’ve seen this same language mix-up many times.)
I believe you are referring to WIDER tires here. Yes, that will help braking a bit. It also makes the car much more prone to hydroplaning in the wet.
Why?
Hmm… maybe it’s a case of [what’s that figure of speech, where the name of the part becomes the name of the whole]?
When you have a blow-out or otherwise lose a tire, you are said to be “rolling on the rims” because you ARE – you’re rolling on the rim (outer edge) of the wheel. From there, you get the whole wheels referred to as “the rims”.
The hub is the bit in the dead middle of the wheel where it gets bolted to the axle, Originally “hubcaps” literally covered only that part of the wheel assembly. Modern wheel covers, of course, cover the whole thing.
Manufacturers, accounting for better suspensions and body rigidities, have been more generous with their wheels/tires. I can still remember as late as 1999 Toyota would sell its Camry 4 with R14 wheels/tires, while the V6 had much better-performing R15s. And I think they’re on 16’s by now, I could be wrong. But the key is to stay within a certain design range – after all if the maker ships it with R15s, there’s probably a reason they didn’t make it R19s. IIRC, in a recent issue of CR they essentially advised that aftermarket wheels (the manufacturer fixes up the suspension accordingly if they offer different wheel sizes from stock) add no more than 2 inches (and they’d prefer only 1", but hey, it’s those bores at CR 
) above stock, at which point you start having a problem of ride, of handling in wet weather, lower tire life, less resistance to physical hazards, and higher risk of damaging the wheel and tire if hitting potholes or curbs
May I say that whatever the ride or safety effect, the mere stupidlookingness of a Hyundai with 18" chromed mags should ddissuade people from attempting these things.
Speaking of tires, did anyone else notice that tires are measured in **both ** inches and metric?
Actually, I hadn’t gathered that. I thought maybe that in fact the rim was the tread part of the tyre.
Just goes to show that I really don’t know a thing about cars. 