I believe they are 4.52" and are 8" wide rims Which basically puts them at a 4.5" backspacing and a Zero offset.
The 20's have the 5.75" backspacing(19mm offset) because they are 9" wide.
I like THIS link for wheel comprisons much better.
So, on the chart, you want to be between an 8" wide wheel that has a 4.5" backspacing(zero offset) and a 9" wide wheel with a 5.75" backspacing(19mm offset)
Anywhere in the middle of those 2 values on the chart will work for your application.

 

Well, with a quick measurement I figured the backspacing is right around 6". The width was difficult to get as the wheels have tires on them, but I think they are 8" wide.

 

Hey no problem at all. The stack of wheels are sitting in a pole barn just a few 100 ft from me... wasnt out of my way much.

 

I think your mistaken. Aint no RWD truck rim that has that much of a backspacing(offset).(6") You would rub the iside sidewall with those wheels if they even cleared the calipers? 20's have 5.75" but 17's DO NOT.
You need to measure from the hub surface to the outer edge of the rim. Doing that measurement with tires on is NOT easy and will make your backspacing larger than what it really is.
Also, when you measure the depth from the outer lips, the width AND height are usually 1" larger than what it really is. So an 8" wide wheel measuring from outside lip-outside lip will measure 9". Just to give you haeds up...

Forget about what backspacing you have because it means nothing! Go with what everyone knows will fit. A 295 is absolute maximum for an 8" wide wheel and you really need to offset the backspacing to move the wheel to the outside justa little so you don't have any rubbing issue's or clearance problems when turning. Also, when you go to a wider wheel, the backspacing/offset all changes so a 6" backspacing is obviously larger on a 8" wheel than it is on a 9" wheel.

 

Nope, not mistaken. It may be more like 5 3/4", but its all of that. I was taken a "quick" measurement, as I only had roughly a 12" piece of wood to put against the back of the wheel, so there was a little eye-balling there LOL.



I am well aware. My old 15x12's (for my 2nd gen) actually measured 16x13 from the outside. A true wheel measurement is actually from the inner portion of the bead, not the outer (which you are well aware of, I was just stating that).

 

Since pictures are worth a thousand words, heres a few pics of my factory 2006 rims. They are not the chrome steelies, but the polished aluminum (although they have the same width and backspacing to my understanding). They are 8"' wide with 6.25" of backspacing...

wheels003.jpg?t=1301451576



wheels007.jpg?t=1301451690

 

That is not 6.25" backspacing there bud. You had it right in your last post at 5.75" You never took off 1/2" for the lip.

The MC/2500/3500 Rams must have diff backspacing because the 1500's are not that much. They are what I stated earlier.

 

Actually, that lip is only about a 1/4". I thought backspacing measured from the outer edge of the wheel, or is only the outer edge of the bead seat? Regardless, its all of 6".

I think the 3/4 ton axles (and half ton MC) are a little wider than the half ton stuff. Also, those 20's on the half tons seam to stick out just a little more than past the fenders than the 3/4 tons, as well as the 17" half ton wheels.