Fitment question

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SR77

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72 Demon, large bolt pattern, 8 3/4 rear end, disc up front, drum in back (switching to wilwood or other upgraded disc all around next year).

Looking at a set of 17x7 +38 for the front, 18x8.5 +25 for the rear.
The rear is stock tub and previous owner half assed the fender roll front and back.
If this will work what size tire?
 
Before I go too far, if you're planning brake upgrades you may want to wait on wheels until you've either done the upgrades or at least decided for sure on what brakes you'll be upgrading to. The reason for that is that most aftermarket brake kits for the front and rear change the track width. Some more than others. The other thing is that in the front different brake kits also have different hub bore requirements. A lot of the wilwood kits now require a 3.050" hub bore, which is huge, so not every wheel choice will fit it without modification. You'll definitely want to know what the hub bore has to be before you order wheels, especially if you need a hub bore larger than 73mm (2.87") which is a fairly standard aftermarket size.

Now, having said that, let's take a look at the rims you're talking about. In the front the 17x7's with 38mm of offset should work. That's a 5.5" backspace, and that is less than the 5.6-5.7" backspace where you start to get outer tie rod interference with some rims. The problem I see is what tires to run. A 245/45/17 isn't supposed to be mounted on anything narrower than a 17x7.5, 235/45/17's have the same rim width problem, and a 235/50/17 is 26.3" tall, which is kinda tall. A 225/50/17 would work, but that's not a ton of tire considering you can probably run a 255/45/17 with the right rim and backspace. On the bright side, you wouldn't have to worry about different brakes increasing the track width some because you'd have plenty of room for that.

In the back an 18x8.5" +25 works out to a 5.7" backspace, which is too much for a stock width A body 8 3/4 and stock spring locations even with BBP axles. You need to end up around 5" to 5.25" of backspace, if you did that you could run 255/40/18's. If you went to an 18x9 or 18x10 you could run 275/35/18's, especially if the quarters have been rolled.
 
Awesome info, thanks. So a 18x10 +25 would work?

Nope, not what I said.

An 18x10 can work, but you have to get the backspace right. With the stock spring locations, the absolute most backspace you can run is about 5.5". That's because with an A-body 8 3/4 and BBP axles and brakes you should have about 6" from the wheel mounting surface to the springs. Since the section width on a 275 is about 11" (same as the outside to outside measurement on an 18x10), you don't have much tire overhanging the outside lip of the rim so that can work. That would mean using an 18x10 +0 though.

Realistically, you'd be better off with an 18x9 or 18x9.5, as that will squeeze the tire a bit. A 275/35/18 is about the limit with the stock spring locations on Demon/Duster, although if the quarters have been rolled you may have some extra room compared to stock. For an 18x9 or 18x9.5 you'd be looking for a +0 to +6, which works out to a backspace between 5" and 5.25" for the 9" rim (like I said above), or 5.25" to 5.5" for the 18x9.5.
 
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