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Discussion Starter #1 (Edited)
I have made my calculations and believe a set 17x8", 4.5" backspace, 255/50 will just fit in my rears perfectly. Maybe 1/2" to spare on each side.

Now I found a deal on a set of 17 x 9", 4.5" backspace (5.5" "front" space). I would put the same tires on them. Are they going to have 1/2" to spare on each side also? Or are they going to set out 1/2" or 1.0" farther from the frame? Or both??? LOL
 

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I have made my calculations and believe a set 17x8", 4.5" backspace, 255/50 will just fit in my rears perfectly. Maybe 1/2" to spare on each side.

Now I found a deal on a set of 17 x 9", 4.5" backspace (5.5" "front" space). I would put the same tires on them. Are they going to have 1/2" to spare on each side also? Or are they going to set out 1/2" or 1.0" farther from the frame? Or both??? LOL
If the wheel is 1" wider, and the backspacing is the same, it will sit 1" further out. Now if your clearance measurements were from the bulge in the tire and not the wheel then you might lose some width in the tire, but the center of the wheel will still move out 1".
 

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Discussion Starter #3
If the wheel is 1" wider, and the backspacing is the same, it will sit 1" further out. Now if your clearance measurements were from the bulge in the tire and not the wheel then you might lose some width in the tire, but the center of the wheel will still move out 1".

Yes the clearances were taken from the bulge in the tire. So with the new wheels will I have 1/2" more clearance on the inside? Or 1.0"?
 

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Yes the clearances were taken from the bulge in the tire. So with the new wheels will I have 1/2" more clearance on the inside? Or 1.0"?
Well the wider wheel might pull the sidewalls in some, but that is all of the additional clearance you will get inside.
 

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Discussion Starter #6
The tire to framerail clearance will remain essentially the same..The tire to fenderlip clearance will decrease by an inch.

That's what I figured! But the tire is the same width, so how is it possible? crazy...
 

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I have made my calculations and believe a set 17x8", 4.5" backspace, 255/50 will just fit in my rears perfectly. Maybe 1/2" to spare on each side.

Now I found a deal on a set of 17 x 9", 4.5" backspace (5.5" "front" space). I would put the same tires on them. Are they going to have 1/2" to spare on each side also? Or are they going to set out 1/2" or 1.0" farther from the frame? Or both??? LOL
I'm used to thinking of wheels in terms of offset rather than back spacing because then you don't get hung up in considerations of the thickness of the rim at the lip.

The wheel width (9in in this case) is measured at the bead seat (inside the rims) while back spacing is typically measured at the outside of the rim lip. The rim lip thickness is typically around 3/8-1/2in from the bead seat. So a 17x8 with 4.5in back spacing is zero to slightly positive offset, a 17x9 with 4.5in back spacing is around 1/2in negative offset.

So those 9in wheels will move the centerline of the tire out 1/2in, plus the tire gets about 0.4in wider mounted on the wider rim of which 0.2in goes to the outboard side, so if you really did have only 1/2in clearance on the outside the tire's now rubbing.

17x9 with half an inch of negative offset is a bit of an odd duck, and you certainly wouldn't want to run something like that on the front, what's the original/intended application of those wheels?
 

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I have made my calculations and believe a set 17x8", 4.5" backspace, 255/50 will just fit in my rears perfectly. Maybe 1/2" to spare on each side.

Now I found a deal on a set of 17 x 9", 4.5" backspace (5.5" "front" space). I would put the same tires on them. Are they going to have 1/2" to spare on each side also? Or are they going to set out 1/2" or 1.0" farther from the frame? Or both??? LOL
Wouldn't a rim with 4.5 backspacing and 5.5 frontspacing(offset)be a 10 inch rim?..Since backspacing is the same you would be adding an extra 2 inches on the front side where you only had 1/2 inch of clearance..The wider rim may help straighten up the sidewall bulge a little though like BH107 mentioned.
 

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Discussion Starter #9
So those 9in wheels will move the centerline of the tire out 1/2in, plus the tire gets about 0.4in wider mounted on the wider rim of which 0.2in goes to the outboard side, so if you really did have only 1/2in clearance on the outside the tire's now rubbing.

17x9 with half an inch of negative offset is a bit of an odd duck, and you certainly wouldn't want to run something like that on the front, what's the original/intended application of those wheels?

I was figuring 8" or 9" wheels, the fattest part of the tire would be the same. Good thing I asked.

The wheels are off a rod build gone bad.

One more question, would a 9" wheel be any tougher to change than a 8", everything else being the same?
 

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Wouldn't a rim with 4.5 backspacing and 5.5 frontspacing(offset)be a 10 inch rim?..Since backspacing is the same you would be adding an extra 2 inches on the front side where you only had 1/2 inch of clearance..The wider rim may help straighten up the sidewall bulge a little though like BH107 mentioned.
A 9" wheel is actually ~10" wide. Wheel widths are measured where the bead of the tire sits (inside wheel lip to inside wheel lip).
 

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Just for perspective and comparison, I have a 66 custom 500 4 door and I'm running a 18x8.5 with 4.25 BS (-25 mm offset) wheel up front and 18x10 with ~4 1/8" bs (-35mm offset) rear. Front tires are 225/45 and rears are 255/45. I am running 2.5" lower than stock springs and have minimal clearances in rear between the tire and QP. Even less tolerances in the front in tight turns.
 

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I am shocked, I thought the 66 had more space back there than the 65's. How much clearance do you have inside, to the frame?
 

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Just for perspective and comparison, I have a 66 custom 500 4 door and I'm running a 18x8.5 with 4.25 BS (-25 mm offset) wheel up front and 18x10 with ~4 1/8" bs (-35mm offset) rear. Front tires are 225/45 and rears are 255/45. I am running 2.5" lower than stock springs and have minimal clearances in rear between the tire and QP. Even less tolerances in the front in tight turns.
Those are fairly narrow tires for the wheel widths you're running, I'm setting up the '64 Country Sedan with the same wheel widths you're using but 245/50 fronts and 285/40 rears. I'm surprised to find the later cars have less clearance than the early tubs. -25mm is a fair bit of negative offset in front, any adverse effect on the steering?

I can't remember the exact offset I specced for the wheels, it's very slightly positive, I pulled the fronts in as close to the control arms as I could. There''s plenty of overall clearance to the fenders and frame except at the lip, I had the fender lip flared as much as I could for clearance without having to remove the side spear but it may still have some impact on on steering lock, and of course that car rubbed the tires on the sway bar even with G78-14s so I'll deal with that when I get to it. I think '64s have the lowest front fender openings of any of the early cars.

Out back it's tight but livable on the outside (there'd be lots more room if one wanted to cut up the fenderwell, which is well inboard of the sculpted outside sheetmetal of the car, but I don't), the bumpstop bracket on the inside's kinda in the way but it's coming off anyway...as you can tell it has little use with the rear suspension that's in the car:



The tires are not as close to the frame rails as they look in that pic:



Never mind all the rough edges on the brackets and crossmember, that will all get cleaned up when the finish welding's done, right now working on engine/trans mounts (and getting the '65 Mustang running, and a bunk bed for the daughter for Christmas...)

The biggest possible frame-clearance issue beyond the bumpstop bracket is that I may have to notch the outside of the frame rails ~1.5in for upper control arm clearance.





 
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