QA1 Suspenssion 335 tire?

[keefro]

I've searched the forums but have not found very good results.

The pictures below are of my 1971 GMC with stock 12 bolt rear end, QA1 coilovers front and rear with a 4/6" drop. I'll have some custom 20" wheels made and am trying to decide on tire size. The 335/25R20 installed on the wheel fitment tool is off my Z06. I'll run a taller 35 sidewall at a 29" diameter and the tubbed inner fenders (wood bed makes it easier).

The speedway motors tool says I can run a 20x12.5 with 7.75 backspace and +25 offset (1").

Math:
(12.5" wheel + 1") / 2 = 6.75
7.75 - 6.75 = +1" or +25

Has anyone gone this wide?
12.5" wheels?
+25 offset?

[CC69Rat]

If you measure from the inside of one brake backing plate over to the other side in the same place. Is it 57" or 55.5"? I'm betting 57".

I usually run 20x10, 6" backspace on the 55.5" axle. At 4/6 or 5/7 static it will clear 295/45 no problem. 295/40 if that's your preference.

Of course this would change if you have the wider rear axle. (Late 69, on up )

My only thought would be the outboard rear coilover mount, I'd be really cautious there .. but I feel like it would work (?)

I'm considering the same setup on my 67 to be honest.. curious to see how it plays out.