My "new" '74 Duster- or why I need a project like a hole in the head

I want a 275 front tire bad.

But when I step up to an 18x9 wheel I will probably start with a 245/40R18 and see how they fit is on my car first. Partly because I see the 245/40 all over the place used on Marketplace and I have seen exactly one 275/35R18. So something cheap that I am pretty sure works seems like a better gamble than jumping right to the 275 and maybe needing to massage some sheetmetal right away. I expect to do some massaging at some point, but nice to take it in smaller chunks.

The other reason I am looking at a 245/40 first is the source for 35-38mm offset wheels seems really small. I found an 18x9 +30mm wheel I like for a price I am comfortable with and figured I would start there. Worst case, having the wheel widened 1/2" in the back would push the offset back to about +36mm. Can't say it would be cheaper than buying a wheel with the right offset to begin with, but it's just an idea right now.

The other factor I have to be careful of is that I need to make sure whatever 18" wheel I pick will work with my homebrewed Brembo kit. Most manufacturers don't seem to have much to say about it, so I have been focusing on wheels that might fit a 2009 GT500, which is why I am leaning towards the AM 2010 GT500 wheel. Others might fit, but not ready to order one and hope I can send it back if it won't work.

Add that my hubs are already machined to match a Mustang hub (outside of issues with the depth of the hub) and it makes some sense to be looking at wheels that would fit a GT500.

18x9 +30's should be fine for the front. Your brake set up should be about the same as mine for track width, mine are +35's but it's a small difference. The other thing is to look at 18x9.5's, it seems like there's a lot in the +30 to +40 range for aftermarket wheels in 18x9.5". And that should be fine too, I'd lean more to +35 with those. If I buy another set of wheels for the front they'll probably be 18x10's.

Really the 275's aren't that big of a deal as long as you've got 6" of backspace. Maybe a trim on the corner and a little lip rolling, the rest is ride height dependent and most folks don't run as low as I do.

245’s can be run pretty easily on the Duster’s. I’ve got 245/45/17s are on my Duster. I’ll have to measure to my wheel opening to compare to Brian’s Duster, but I’m running an HDK slammer suspension so I’m pretty sure it’s every bit as lower if not lower. I didn’t do anything to the fenders when I had it lowered on 1.03 torsion bars, but the HDK ended up dropping it at least another inch and the tire would rub the lip at full lock, so I ended up rolling the fender lips.
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I guess here I had it at 23.5” but I think I raised it up some due to the tires hitting the bottom of the inner fender when I hit decent sized bumps.

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Yeah the suspension isn't the issue for me in going any lower, the wheels hit the body and that would be true of any kind of suspension up front. Maybe even more so with the HDK, the way I'm set up with my QA1 LCA's and bumpstop I should be on the bumpstop at about the same time my tires touch the inner fender. Apparently there is a little bit of overlap there...
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This will be my next major bodywork modification, I'll add the tub to raise the inner fender (72BBswingers old ride)
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I'm torn between 17s or 18s. I'm thinking 17s, 245/45 (or even 235/50) probably. I was thinking 4.5" backspace on an 8" wheel...you say 5+"?

Yeah, that's too low. I would lose headers in a week that low in this area.

Yeah an 8" wheel with 4.5" backspace isn't enough. For a 245 the minimum would be about 5.25" of backspace, and that would put the outer face of the wheel in the same place as my 275's. Ideally you'd want more like 5.5"-5.7" if you wanted to avoid doing any trimming/fitting. With a 17" rim ~5.7" is the maximum backspace too, just FYI. A 17" rim doesn't clear the outer tie rod end much beyond that. With an 18" rim the outer tie rod end fits inside the lip of the rim, so you can run more backspace (6"+) which is what you need to step up to 275's.

My Doug's headers sit around 3.75" to the flange on the low side. It's not bad as long as I take it easy on speed bumps.