Help me confirm my measurements and back spacing thought

69_340_GTS said

If you take 4 7/8" + 7" that gives you 11 7/8" total fender lip to spring spacing, at the most. Your 275 tires are about 11" wide at the sidewalls. Not including a margin of error, sidewall flex from cornering, side-to-side body tolerances, etc... So you have 7/8" total clearance, or 7/16" on each side at the most. Did you measure both left and right sides of the car? They are usually different. It doesn't matter what backspace wheel you put on it, as far as I'm concerned a 275 tire just isn't going to fit without some kind of body surgery.

{By the way, by my figuring, the theoretical backspacing for your car should be 5 9/16" (or 1 1/16" offset) with the 8" wheel. Same as what 72bluNblu came up with.}

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Yeah an even split won't work, but you can run the springs a little tighter. 1/2" at the quarter and 3/8" to the springs. It has to be perfect though, because if you miss either of those measurements it will probably rub. And good point about adding the reminder about the sides not being equal, because they usually aren't and even an 1/8" difference in this case would mean trimming something. That's why I would get more backspace than required, so I could adjust the final backspace with a thin spacer.

Most of the Darts that run 275's do have some work done to the quarter lip, because realistically none of these cars are perfect, and although the math works the actual execution is obviously what matters. And with a 275/60/15 you need more space for sidewall flex than you can get away with if you're running 17's or 18's because the shorter sidewall doesn't allow as much movement. I run 17's and 18's so I may be selling the sidewall flex on the 275/60/15's a little short.