My car is completely stock, aside from replacing rotors and pads.
Wondering whats the widest 18 inch wheel that will fit?
Considering overall diameter of the wheel/tire combo, I'm figuring I would be about 40mm max and as close as maybe 5mm off the stock overall rolling diameter. How will this effect the AWD and ABS?
Lastly, brake temperatures. Wondering if by running a smaller wheel, will the brakes get less cooling? And how much less. Wondering if ducting would have to be implemented.
For 18" wide wheels see post #16http://www.gtrlife.c...heels-wont-fit/
AWD and ABS will be fine if you can keep front and rear tire diameters very close 1-3%. Another solution is to use the same tire size on front and rears with either the same size wheels or slightly wider wheels in the rear but choose wheels that can fit the same tire size.
In general if you can increase width of the front wheel and or tire compared to stock, you can reduce understeer, The biggest problem in the front is using a 18" wheel that can fit over the large front brake caliper or clear the inside suspension parts if offset is not aggressive enough.
If you choose a tire size that is smaller than stock you can lower the center of gravity even with stock suspension but be careful about going too low since there can be risk of scraping the front lower lip area on tracks that have dips or rises if taken at higher speeds. Tire diameter as low as about 25.5-26" can be used with stock suspension vs OEM 28".
Brake cooling with a 18" can be slightly reduced. If you upgrade brake pads and brake fluid that will help. Brake ducting is also helpful. It is also possible to use a wheel spacer in front depending on how wide the wheel is to increase a little space for cooling. A wheel stud that is longer than stock will help when using a spacer.