Help me confirm my measurements and back spacing thought

autoxcuda said

So it's an original A-body width rear end. Just big bolt axles installed?

The inner wheel well curves in. When the car leans to the side, the top of the tire tilts in relative to the body.

I have a 68 Dart that has the same wheel lip as your car (same wheel lip molding). I've got 245/60/15 15x7 rim with 4.25 backspace and small bolt axles. It barely touches on the outside and inner wheel well top and the springs. Springs stock location.

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Right, and a 15x7 with a 4.25" backspace has the same centerline as a 15x8 with a 4.75" backspace. But he has BBP axles so add 5/16" for the increase in track width. And he has a 3/4" offset so add another 1/2" (yes, I know I added a 1/2" but even the 3/4" offset only offsets the front hanger a 1/2") . Now the backspace is at 5-9/16".

I run my duster with a 1/2" offset and about 5/16" of clearance to the spring, it just barely touched the inner wheel well when I was cross-axling speed bumps before I added the rear sway bar. Inner tub is the same on both of those cars. He should be able to manage 5 5/8" with the measurement he has. I wouldn't run the spring clearance that tight on a 275/60/15, my 295/35/18's have like 2.5" less sidewall to flex.