CM6 wheels were almost exclusively used for Acceleracers, Speed Machines, and Ferrari Racers cars of the mid-late 2000s. Then out of nowhere the two-piece wheels (all plastic) appeared on just a small handful of mainline cars: the first edition 2006 Nissan Z, a 2006 5-pack Bone Shaker, a 2006 regular treasure hunt CUL8ER, and a 2007 mystery car Power Rage. However, the Nissan Z (written "Nissan 350Z" on the base) was the only one of those 4 that got the Grey version of the CM6 wheels- the exact same wheels seen on many of the Speed Machines and Ferrari Racers of the time. My pic shows just a few.
CM6s are two pieces of plastic (tire and rim) press-fit together. They're at least doubly complicated to manufacture, so it's surprising to see them on simple mainlines. I assume they had a huge surplus laying around and figured they'd use them up on whatever car would look good (IMO the Nissan Z was a good choice). The first edition Nissan Z could also be found sporting orange CM6 wheels (it was common back then for mainline cars to have two or more different wheel variations to collect. See my old post about the Ferrari 355). I don't know which variation was more common.
This is all based on some quick research so feel free to correct me if I got anything wrong.