back spacing

I have an 07 nbs silverado with factory 20s, want to purchase custom 18s or possibly 17s, what is the proper backspacing I will need for this?

Welcome to the forum!!! Hang in there, someone here should be able to answer your question.

donny- im not sure what the proper one is but it is easy to figure out your backspacing so you can pick the wheels and tires you want. just make sure to account for the tire bulge on the backside of the rim, good luck
http://www.bigblockdart.com/techpages/wheelspacing.shtml

ok, thanks for the info