I called my self trying to calculate back space and did not do it right. I was using the wheel that was on the car with out taking it off. I thought it was 4 inch back space with about a inch to spare. I bought some intro pentia 17x10 4.5 b/s rear and 17x8 4. b/s front, I ask for this size. After a day of slobing of the wheels something told me " Hay nut try the wheels on the car. I just pulled the old wheel off the back and found out it has a 3.5 back space. Then I noticed ,oh look a brake cable. Where did that come from?. New wheels do not fit The rear. Never trid the front. Only one good thing, the wheels only cost me 750.00 with rolled lip.

I guess I have a case of the blind leading the blind. That donut hole told me the were  4.5 rear and 4 front and I just found out the are 5.5 back space rear and 5. front. damm dammm dammmm dammm

Youy can try VBP & get some spacers. Call them first

Ok guess im all smiles now. I called intro that makes pentia's He told me that the 17x10 comes standard with 5.5 back spacing. but they have put them on hundreds of vettes. He said just move brake cable and I will have to get a off set sway bar and it will bolt right on. He said I can also use spacers using Et lug nuts. If I use a spacer I can run a 295 size tire on the 10 wheel. what do you think? 

I;ve been reading a lot about wheels lately, and I've yet to see this answered (or asked). How do you measure the backspacing you have/need for your current wheels? If you get larger wheels, can you get ones with the same backspacing and be ok (within reason - something like 17" wheels)?

Offset:
The distance from the centerline of the wheel to the wheel bolt up face, and is described as Positive or Negative.

Positive Offset describes the bolt up face and outside of rim is farther towards the outer part of the wheel.

Negative Offset Offset describes the bolt up face and outside of rim is farther towards the inner part of the wheel.

Backspace:
Backspace is the distance from the edge of the inner rim to the wheel bolt up face.

The width of the rim determines the centerline and from there you can calculate backspacing and offset:

Complete Info Here

Example: On a 10" rim, centerline is at 5". A 4.5" B/S is 5" - 4.5" = .5" positive offset (outer rim face away from the bolt up face). Changing to a 4" B/S is 5" - 4" = 1" positive offset. So while a 4.5" B/S pushes the rim out .5", a 4" B/S pushes the same rim out 1".

Say Jimmy, I thought the spacers had the studs on them. You bolt the spacer down using your studs and use a et bolts. They screw on flush. you bolt the wheel on to the spacer studs.