Rotating mass. This was the first thing I noticed on my other car when going from 17 inch to 19 inch wheels. Even though I maintained almost exactly the same diameter by tire selection my braking was compromised. There were some other affects as well that few probably consider. Even if you maintain the same overall weight of the package what can be an issue is relocation of the rotating mass from the centerline. This can be an issue even if maintaining the same size and weight wheel.

 

Holy smokes, you are killing me people. I just ordered some 19" for my G37X sedan. . Wheels are 21lb each (I tried to keep weight close to 17" OEMs). Actally I don't know weight of 17" OEMs but I don't think that they are lighter than 21lbs.
From this point my car will have beauteful wheels with 2 inch gaps everywhere.
I'm definately not lowering

 

Maybe not two inch gaps. Keeping the weight the same may have helped, what is equally important is the location of the rotating mass. Consider the difference between two common ferris wheels which weigh the same and have 90% of their weight at the location of the seats. Which one is harder to stop, one that is 20 feet in diameter or one that is 40 feet in diameter? Now move that 90% of the weight to the location of the hub on the 40 foot one. Which one is harder to stop?

Did you select tire size in order to keep the same diameter? That is why I selected 19s on my other car to replace OEM 17s. the result was almost exaclty the same diameter, unfortunately I can not claim the same benefits for brake performance.

 

Just put on some new 0Z ultraleggera 18x8" with 245/45's weighed all my old oem 17's with approx. 50% tread left,, 47 pounds,,, then put all my new rims with Kuhmo ecta's on the scale.... 47 pounds!!!!! each.. i am very pleased with this summer combo..... the 0Z's are 18.4 lbs each,, so the rest was the beefy Kuhmo's.....

 

Yes, I selected 245/40/19 which is 99.9% of stock 225/55/17.

I have one more question to people who clearly have much more knowledge than me. There are two AWD sedan models - G37X (17") and G37xS(18"). Both are AWD but "S" has 18" wheels. Does it mean that "S" model has worse stopping distance than non-S model?

 

No,, the XS version is diff. in these ways,,, better sport seats, aluminum pedals, paddle shifters, (auto) front bumper valence diff. 225/50/18's all the way around,,, compared to the 225/55/17's on a regular x.... the brakes are identical on the two diff. X models.. the xs, may handle a bit better with the 18's... plus imo,, those xs wheels look nice.....