Wheel dimension help needed

[Tri5Nerd]

I have a narrowed 9" on my 55 second series, the width from outer flange to outer flange is 58", after measuring I got 6" from the center of the wheel to the inner lip of the rear fender. So I would subtract 1.5" from that in order to get clearance once the truck is aired out.

If I was to run a 20x10" rim I figure I can have up to 6" of backspacing, if so then my offset would be +12mm? Is my measurement correct?

BTW: I use some information gathered from this site

Tri5

[SCOTI]

BACKSPACING AND OFFSET
Backspace and offset are two ways of describing where the wheel sits in relationship to the hub. As such, there is a relationship between the two making - it possible to convert from one to the other.

Backspacing is the easiest to understand. It is simply the distance from the back of the mounting pad to the back lip of the wheel. It is measured by laying the wheel on its face, placing a straight edge across the wheel, and then measuring down to mounting face. The higher this measurement the more the wheel is pushed inboard towards the car’s centerline, as shown in the illustration above.

Offset is the distance from the mounting surface to the centerline of the wheel, expressed in millimeters. Positive offset means that the wheels are pushed inboard towards the centerline of the car like a car with front wheel drive. Negative means that the wheels are moved outboard towards the fenders, widening the track.


TO CONVERT OFFSET TO BACKSPACING:

For positive offset wheels: (Wheel Width +1)/2 + (offset * .03937)

For negative offset wheels: (Wheel Width +1)/2 - (offset * .03937)

TO CONVERT BACKSPACING TO OFFSET:

Backspacing – (Wheel Width + 1)/2 * 25.4 = offset

Round this result to the nearest milimeter. If the answer is a negative number the wheel has negative offset, if positive then a positive offset.

[Tri5Nerd]

So then I could run a 20x10 with 6.25" and backspace of .20mm offset, would that fill my fenderwell and allow me to airout?

Tri5

[SCOTI]

6.25" back spacing is a bunch so it's going to hit somewhere. Somewhere could be the a-arms, inner fender sheetmetal, a/c box..... You'll need to verify your exact application for accurate information.

[Tri5Nerd]

Originally Posted by SCOTI

6.25" back spacing is a bunch so it's going to hit somewhere. Somewhere could be the a-arms, inner fender sheetmetal, a/c box..... You'll need to verify your exact application for accurate information.

I should not have an issue because I inner fenders are tubbed and I have 9" from the rearend outer flange to the outside of the frame.
The rim will need to have somewhat of a deep dish to fill the wheelwell.

Tri5