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.

[keefro]

Thanks for the response. I'm new to these years and forgot about the difference in track width with the disc brakes. My 71 was originally a leaf spring rear end that I converted to track arms with coilovers.

When I measured base plate to base plate, it measured just over 57".

The wheel fitment tool allowed me to check the clearance on the Coilover. It was just over 1" away with the 7.75 backspacing.

I think what confused me was people talking about the 6" backspacing and me not considering the difference in axle width.

I may end up doing 8" backspacing and run a spacer to give me some optionality on tire width.