Despite the fact there is hundreds of posts on this topic, I still cant find the answer to my question. I have a set of 4 18x9.5 c6 rims with a .56 offset.
I understand you need to calculate the backspacing to find the spacer size. With a max backspacing of 5 in the front and 5.85 for the rear.
http://www.skulte.com/adapterfaq.html
Using the Info on this site I come up with this...
9.5/2 =4.75 +.56 + .25 =5.56
So with a max of 5 wouldn't I only need .56?? And shouldn't the rear wheels clear completely by these numbers??
I'm just trying to understand why I keep hearing 2in spacers.. Am I missing something?

 

Are you sure its not +56mm (millimeters) offset? Offset is usually (always) provided in the metric system. 25.4mm per inch.

John

 

Yes thank you, that was the issue.
so with that being said
4.75 + (56/24.5 [2.20]) + .25 = 7.2
So 7.2 - 5 =. 2.2
7.2 - 5.85 = 1.35 for the rear
So most people seem to run 2 in the rear and 2.25 front? Would the quarter inch offset be that noticeable? Because ideally you want the rims to appear even ( distance out from the car)

 

I think 1.35 in the rear puts the wheel face in the stock location but you need 2 inches to have the wider vette rims clear the inner fender

 

The stock setup wasn't equal anyway, the rear faces were farther in, and look really bad. And you only need 2" front spacers with most big-brake upgrades. Or go thick for now, then have the spacers milled when you do the brakes, because 10.5" rotors look as bad, in 18" wheels, as they "perform" in any wheels.

 

First, wheels actually measure about 0.5" wider than the stated width for the lips on both sides, so a 9.5" wheel = 9.5" + 0.5" = 10.0".

Here is how backspacing works: take 1/2 the total wheel width as the 0 offset baseline: 10"/2 = 5". Then you add 56mm (2.20") to the 0 offset to get actual backspacing = 5" + 2.2" = 7.2".

Since 3rd gens typically use 0 offset in a 9.5" wheel 9or about 5" backspacing), you need to use a 2.2" spacer (2.25" is available) to clear the front suspension, rear is more forgiving. If you have a big brake kit on the front, you may be able to use 5.25" backspacing as a baseline.