What back spacing or offset do I need

[D-Rock]

Hi guys, I have a 79 k20 that I'm restoring. The truck already has 6" springs on all 4 corners and I'm considering a body lift also for clearance. Currently it has 35x12.5/16.5" and I want to go to a 17" or 18" wheel instead of the 16.5" for more selection of tires. I'm wanting to have it look era correct so that limits things but I'm wanting to know what backspace or offset I will need, I don't want them to stick way out but I will be running 35"-38" tall and 12.5"-15.5" wide. What do I need on my wheels?

 

[Grit dog]

You need to decide on a tire size first. Can’t pick a 1 size fits all “backspacing” for a range of 3” in height and 3” in width. And can’t look “era correct” (presume you mean wrt how far the tires stick out) with 14-15” wide tires.
And 37s are sketchy and 38s a hard no with only 6” of lift. Especially if you’re going wider than 12.50.

But for a relevant example, these are new 35-12.50s on 9” wide wheels with a 3.5” backspace. Stout 4” lift and these rub at full lock. I need more lift/move axle forward due in part to the offset.

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[Grit dog]

Friends don’t let friends do body lifts btw….

 

[BKING33]

I had 36" Swampers on a previous truck I had. They were 14.50 wide. I had them on a 12" wide wheel with 4" back spacing. I had 2.5" fender flares on it. With a wide base tire you will need about 8" lift and notch frt fenders at the bottom.