17's vs 18's WAIT this one is different!!

[Stu Seitz]

Like the title says I’m trying to compare a 17 inch to an 18 inch wheel but with no change in rotating diameter. I know most people think that a 17 inch wheel is more advantageous on a road course because of its smaller rotating inertia, but is that only true when it’s compared to an 18 inch wheel with a larger rolling diameter?

For example a 275/40 17 is actually a smig bigger then 275/35 18, on top of that I have held both tires and the 275/35 18 even feels lighter and I know for a fact that a 285/30 18 is lighter then a 275/40 17. So if one were to just change the diameter of the rim but not the rotating diameter of the wheel, one would think that there would be no ill effects on performance?

Just thinking out loud, so please feel free to correct me if I’m wrong.

[Ralph LoGrasso]

Stuart,

Just FYI, the difference between a 275/40/17 and a 275/35/18 is slight. It's about one tenth of an inch or less, depending on the manufacturers. Theoretically, a 275/35/18 should be lighter considering that both tires are nearly the same height with the 18" benefiting from a smaller side wall (3.78” vs. 4.33”). The same goes for the 285/30/18 (3.36”) which has even less sidewall. However, when comparing different manufacturers and compounds, this changes. If you like the 18s then run them; the performance lost from a set of light weight forged 18” wheels (running good tires with a decent sidewall) as opposed to the same in a 17” wheel will be negligible, IMO. Just make sure the wheels are lightweight, and the tires are good. All of the GT series cars (C5R, C6R GTS-R, etc.) run 18s, nothing to worry about.

[Damn True]

The real weight difference will come not from the tire, but from the wheel. An additional 1" of dia results in a big increase in the amount of material present in the hoop of the wheel and that material is further from the hub. How much material are we talking about?

On a 17" wheel you have a circ of 53.5". With a wheel width of 9" that works out to about 481.5 sq" of material in the hoop. On an 18" wheel you have a circ of 56.5". With a wheel width of 9" that gives you 508.5 sq" of material in the hoop.