I'd appreciate any advice/info on the following:

1. What's our bore diameter? My understanding is that a precise fit at the bore is preferential, but that most aftermarket wheels tend to pick the largest size commonly used on the market to fit as many cars as possible. I doubt there's any difference in bore diameter between Coupe/Sedan/350z or 17" vs. 18" wheels.

2. Bigger offset means further inward from the fenders, right? So if I buy a wheel with 7mm more offset, but about 26mm (1 inch) more width, does that mean I'm going to end up with a net of 6mm further outward toward the fender (half of 26 minus 7)? Is there a number I should be aiming for? I've read that it's usually not a good idea to maintain the same offset when you increase width (as opposed to diameter).

3. I know people use spacers when there isn't a good fit, but the impression I get is that it has some sort of negative effect on the car. If so, why?

4. My Coupe presently has wheels that are 0.5" wider in the rear than in front. If I increase the difference to a full 1" wider, is my car more likely to oversteer or understeer under throttle (compared to stock)? My instincts say more understeer.

Much obliged for any response.

-Jack
Obsidian/graphite '03 Coupe premium nav

 

1. I don't know.

2. Yes on the direction. The number you should aim for depends on a few factors, such as the rim width and brake caliper clearance and suspension/fender clearance issues. Here: http://www.tirerack.com/wheels/tech/offset.htm

3. I don't know. I've heard it has a negative effect too, not sure why though.

4. Correct. More rubber in the rear than in the front make the car lean more towards understeer.

2003.5 G35 Sedan Desert Platinum/Graphite Premium/Sport/Aero/Nav/Winter

 

1. is 66.1mm for all.
2. The higher the offset number, the rim centerlines are moving further inward towards the center of the car. Lower offset numbers are moving the centerlines out away from the center of the car.
3. When you use proper stud length's and fitments, wheel spacers are fine. Yes people do use spacers to make up for not being able to get their desired offsets on their favorite rims. Nothing wrong with that, your being smart by adapting and overcoming.
4. Understeer/oversteer behavior will follow tire contact patch's front to rear. Do follow proper tire width to rim width logic. You will not want to run any rim width that is less than the tread width for that tires given specs. Say your desired tire's have a tread width of 8"'s, run 8" wide rims ect. It's up to your if you want to maintain the 20mm wider rear tire size, I'm inclined to recommend that you stay with that spread and use other means to lesson understeer, via chassis mods.

Ivry 6mt sed
Injen CAI, 22" resonator
8X18 Enkei RPM2 45mm
Kumho MX 245/40
clear corners