I have searched here, searched ls1tech, and searched google trying to figure out what exactly I need before I order it. Right now I know that my front wheels are supposed to be 4th gen offset (they were custom ordered). I measured and found them to have 6.0" backspacing and they are 9 inches wide.

So from there I am assuming that I have a +38.1mm offset. I get that as follows:

9/2 = 4.5 - wheel centerline
6.0-4.5 = 1.5" - offset in inches
1.5"x25.4 = 38.1 - offset in mm

Is my math correct? If so, continue on.

According to the Skulte adapter FAQ, you take the backspacing minus the max backspacing to figure out the size of a spacer you need. According to the FAQ the max backspacing in the front is 5 inches so here is my math:

6.0 + .25 = 6.25 - backspacing including the .25" reccomended by the FAQ.

6.25 - 5.0 = 1.25 - spacer thickness I need.

Is this all correct? I was under the impression that since these are 4th gen spec wheels, that I needed a 2" spacer. But, since these are custom made wheels, does that change it? Why am I getting 1.75 instead of the 2 I thought I should? Someone PLEASE help ASAP so I can order my adapters.

 

I ran a true 6" BS wheel (measured) with a 1/2" spacer on the front of my car, so depending on the wheel, you can run as much as 5.5" BS.

Actually, now I run the same wheel with a C4HD brake setup & a small .100 steel spacer.

I prefer the tucked look of the front tires, as well as the better fender clearance of running them inward as much as possible. Every wheel will have a different inside lip that will require different spacing to clear the tie rod.

You can often mock up the wheel with a stack of washers & an extended shank lug nut (depending on your stud length) to measure the true required clearance. This is the best way if you are patient enough to get the wheels & then order the spcers.

As far as the math, it is a good ballpark, but wheels vary on the lip thickness so it only gets you close. The best way is to measure on the car.

Als check for wheel bearing dust cover clearance to the center of the wheel. This is much thinner on many new style wheels & may change your spacer needs. This is where the limiting dimension is now on my wheels.

 

I didn't check your math because I'm not in the mood to think much but if you are using the maximum backspace to fit the wheels you are tucking them in as much as possible. Most people prefer the look of having the wheels out a bit further so they use a thicker adapter for appearance purposes.

 

I chose to set them in as far as possible so that I can run a 275/40-17 without body/inner fender interference on a lowered car. A 285 only touches at full lock.

If I was running 245's or 255's I would not go through as much effort to set them inward.