1970 dart reared question

lamont 71duster said
So I’ve been trying to see what is the biggest sized tire I could fit under the car and a lot of things I’ve read everyone says something about putting a B body rear end in it to fit wider tire under it.can anyone shed some light on this to help me and explain what u need to do to make it work
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The amount of tire that will fit in the rear wheel tub isn’t really changed by the width of the rear axle as long as the backspace on the wheel centers the tire in the wheel well. The B-body rear axles are just cheaper than A body rears, they don’t let you run a wider tire. The wider rear axle does work better if you want to run a 17 or 18” wheel diameter, but that’s only because most of those wheels come with more backspace off the shelf. On the flip side more 15” rims require a custom backspace for a B-body rear, you’re more likely to fit an off the shelf 15” rim with an A-body 8 3/4.

On a Duster, with the stock spring locations most cars will fit a 275mm wide tire without any rubbing as long as the backspace is correct. The stock wheel tubs are capable of taking a 285 if the springs aren’t in the way, and a 295 if you’re willing to work on the quarter lip a little. That’s possible with a 1/2” spring offset.

If you want more than that, you need to do a full 3” spring relocation AND a mini-tub. The full spring relocation still won’t let you go wider than the 1/2” spring offset does because the stock tub is the limiting factor at that point already.

So the real question is, what size tire do you want to run and what rear axle do you have now?

diymirage said
yup, the early B is about an inch narrower overall, which will allow for half an inch of extra backspacing but all that does is move the mounting surface of the rim around, it does not affect the rim width in any way

moving the leaf springs in does relatively little too, so really, if you want to run a wide tire you're looking at mini tubbing it
(of course, then you're still moving the leafs, but my point is, by ONLY moving the leafs you only gain an inch or so, and the next limiting factor is the factory wheel well, as ATA mentioned)
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All the B-body rears are wider, not narrower. And a 1/2” offset and a little quarter lip work allowing you to run 295’s is pretty significant. A leaf spring relocation and mini-tub is a fair amount of work, and a 295 is a lot of tire for anything less than 600 hp.

Here’s the data on rear axle widths. Even the 62-64 B rears are wider (they’re tapered axles so not a good choice anyway though).


8 3/4" Rear end widths, drum-to-drum:

A-BODY
B-BODY