20's on a Chevy F/S...what backspacing?

Just curious what backspacing you guys are using on your 20's/22's on your fullsizes to tuck.

Ok stupid me forgot to tell the wheel size. They are 20x9.

Yeah that is what I have, I will be putting a 06 GMC front end on it but the fenders are basically the same. I appreciate it man.

I guess while I am asking, what tire size will work on a 20x9. I know I want to stay with a 35 series sidewall because the truck gets driven alot and I don't want to bend a rim everytime I hit a pothole on these crappy Atlanta roads.

I appreciate all the help!