I'm planning on narrowing a 12 bolt rear for my 69 FireBird.

How much clearance for the tire is typically required between the sidewall of the tire and inner rear fender well?

Would 1/2" be enough? I don't want to rub the inner fender sides, but want to maximize space for larger tires.

I'm considering a 59" hub to hub 12 bolt, plus 1/8 each side for the drum thickness. The OE 12 bolt was 59.75" plus the drums.

Has anyone narrowed their FB running a 12 bolt in a non-aftermarket tubbed situation? What width did you go with?

I want to add that after searching everywhere, I could not find this information with this detail.


1969 Pontiac Firebird 8.2 factory rear end (nodular Iron HD) width outer axle hub to the outer axle hub 60.75" (my factory rear I measured)

Brake drums add 1/8" width to each side, 1/4" total or
Disc brake hubs add 1/4" to each side, 1/2" total

1969 Camaro 12 bolt factory rear end width outer axle hub to the outer axle hub 59.75" (same spring pad locations at a 69 Pontiac) per Moser

1970 and up rear axles typically are 1" wider and have different spring pad locations. I have not verified personally.

And this is why those running a chevy 12 bolt can fit 275/40/17 tires in the back, and those with the Pontiac rear cannot in stock configuration.

1/2" sounds tight, but it's also going to depend on your suspension setup and tire profile.

Below is a video of a leaf spring suspension movement,



They had low profile tires (I'm guessing 40 series), now imagine if those were 15s with 50 or 60 series how much additional tire flex you need to consider.

You could reduce the suspension movement, using the delrin shackles, a panhard bar, or watts link.

And you could reduce some play between the differential and the leaf springs by removing the rubber mounting pads, and using a thin (1/4" or so) "lowering block" between the perch and the spring for a solid connection (I have mine setup that way).