15x8 w/3.75" backspace question

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Furious65

Furious65

C-body Guy
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I have some American Racing Outlaw II rims, 15x7 and 15x8 w/3.76 and 3.75 backspacing respectively that aren't being used right now. Can I fit the 15x8's on the rear of my Barracuda without issue? Yes, car is BBP. I would just slap them on and check but they are stored over an hour away and really don't want to drive that far if it's not even close to fitting.
 
You didn't mention offset which affects the fit and appearance, too. Assuming its a "zero" offset, I'm betting you're good with the 15x8 fit in the the rear fender well. A 4" or 4.25" backspace is better. The appearance may be a little "pushed out" but you'll have to mount them to see the look and decide if it's what you want.
 
He told you what the offset is, indirectly, but as usual it has to be calculated, as opposed to BACKSPACE, which is simple, and measurable. Exactly why I don't like that offset bullshit, besides the fact I can't be bothered to figure out which direction is + and which is minus.
15x8, which is really 15x9, with 3.75 backspace is 3/4" offset, or 18 or so mm, and I don't know or care + or-. The b.s. tells me that.
Since I don't know what year barracuda he is talking about.... 64-66 MIGHT fit, with the SMALLEST appropriate tire for the rim width. (Assuming no wheel adapters. If it has wheel pattern adapters,, there is NO way that wheel fits!) A 67-69 should be easier, considering the more open ww opening, IF it doesn't have too much tire on it.
As always, the wheel with the correct b.s. will fit the best (4.5 bs).
 
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Sorry, forgot to include year. It's a '68. I don't plan on keeping these on long term was just hoping I could get my car on some wheels and maybe do a few cruises this summer.

Edit: Offset is -19mm if that helps.
 
What size tire?

3.75 isn’t the best backspace for a stock A-body rear with large bolt axles with thicker axle flanges. Especially with 15x8 rims.

Is the rear end jacked up at all?

at stock height, 4.25” rear backspace on a 15x7 with 245/60/15

275/60/15 on a 15x8 with 4.5” backspace. Might need thin spacers to center perfectly
 
I would use any tire that would fit! I just want to drive it this summer and don't want to spend money on rims that aren't going to stay permanently. I have these rims, bought and paid for back in '97, and they aren't being used right now. Car does have SS springs holding a Strange S60 so it does sit up just a bit but not much.
 
Furious65 said:
I would use any tire that would fit! I just want to drive it this summer and don't want to spend money on rims that aren't going to stay permanently. I have these rims, bought and paid for back in '97, and they aren't being used right now. Car does have SS springs holding a Strange S60 so it does sit up just a bit but not much.
Click to expand...

In the front with 15x7 with 3.75" backspacing you'll need 205/60/15 or maybe 215/60/15 will fit, but close on a 67-69 Barracuda

Since the rear is up with SS springs, it's you're call if wheels are going to hit some big dip and rub the outer wheel lips. Some SS spring set different than others. 245/60/15 might be safe. You'll need to mock up and make a call.
 
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Though the backspace is clearly not ideal, with a conservative tire selection (not too tall, not too wide) you should be okay. Just err on the small side for the tires.
 
On the Back I once installed 275/50-15s, on 8.5s with 3.75 bs; so I think your 8s will be ok. I later installed 275/60-15s on those 8.5s and they still fit...... after I moved the axle back a bit.
My car is lowered some, so they had to be IN the tubs.... at the top. and the front needed a tad more clearance at the rockers.

The 15 x 7s will work on the front as per the recipe from @autoxcuda
 
Terrible backspacing for BBP brakes front and rear.

In the front 205/60/15 is as wide as you should go. They'll already stick out further than 225/60/15's on 15x7's with 4.25" of backspace, which were just about maxed out for size on my Duster. They didn't rub but those 205's with 3.75" of backspace will stick out another 1/8" already. They're shorter though so that will help.

In the back the 15x8's with 3.75" of backspace and a 245/60/15 would be a little further out than the 295's are on my Duster. I needed to trim and roll the quarter lip about a 1/2" to make those fit, but I wasn't running SS springs. So, maybe. I also ran a 1/2" spring offset to get the 295's but with 245's and that small backspace that wouldn't be necessary, you'll have a ton of room to the springs.

33IMP said:
He told you what the offset is, indirectly, but as usual it has to be calculated, as opposed to BACKSPACE, which is simple, and measurable. Exactly why I don't like that offset bullshit, besides the fact I can't be bothered to figure out which direction is + and which is minus.
15x8, which is really 15x9, with 3.75 backspace is 3/4" offset, or 18 or so mm, and I don't know or care + or-. The b.s. tells me that.
Since I don't know what year barracuda he is talking about.... 64-66 MIGHT fit, with the SMALLEST appropriate tire for the rim width. (Assuming no wheel adapters. If it has wheel pattern adapters,, there is NO way that wheel fits!) A 67-69 should be easier, considering the more open ww opening, IF it doesn't have too much tire on it.
As always, the wheel with the correct b.s. will fit the best (4.5 bs).
Click to expand...

Offset is really useful. For example, if you're looking at two different width rims but the offset is the same, you know that the centerline of the wheel is in the same place. So for example if I'm comparing an 18x9 to an 18x9.5 and they're both +35, I know that the centerline of the wheel is in the same place and the 9.5 will add a 1/4" to both the inside and outside. The backspacing will be different, 6.41" vs 6.66".

It's also super easy to calculate, all you have to know is the centerline of the wheel and either the front space or the backspace. The +/- thing is also really easy. 0 offset means the mounting surface is at the centerline. + means the mounting surface is outboard of the centerline, - means the mounting surface is further to the inside. Easy. Both of the rims in question with a 3.75" backspace have a negative offset, -7mm for the 15x7 and -19mm for the 15x8. And that's with the same backspace! So it tells you the mounting surface moved inboard from center by a 1/2", which means the extra 1" of rim width is all to the outside.
 
72bluNblu said:
Terrible backspacing for BBP brakes front and rear.

In the front 205/60/15 is as wide as you should go. They'll already stick out further than 225/60/15's on 15x7's with 4.25" of backspace, which were just about maxed out for size on my Duster. They didn't rub but those 205's with 3.75" of backspace will stick out another 1/8" already. They're shorter though so that will help.

In the back the 15x8's with 3.75" of backspace and a 245/60/15 would be a little further out than the 295's are on my Duster. I needed to trim and roll the quarter lip about a 1/2" to make those fit, but I wasn't running SS springs. So, maybe. I also ran a 1/2" spring offset to get the 295's but with 245's and that small backspace that wouldn't be necessary, you'll have a ton of room to the springs.



Offset is really useful. For example, if you're looking at two different width rims but the offset is the same, you know that the centerline of the wheel is in the same place. So for example if I'm comparing an 18x9 to an 18x9.5 and they're both +35, I know that the centerline of the wheel is in the same place and the 9.5 will add a 1/4" to both the inside and outside. The backspacing will be different, 6.41" vs 6.66".

It's also super easy to calculate, all you have to know is the centerline of the wheel and either the front space or the backspace. The +/- thing is also really easy. 0 offset means the mounting surface is at the centerline. + means the mounting surface is outboard of the centerline, - means the mounting surface is further to the inside. Easy. Both of the rims in question with a 3.75" backspace have a negative offset, -7mm for the 15x7 and -19mm for the 15x8. And that's with the same backspace! So it tells you the mounting surface moved inboard from center by a 1/2", which means the extra 1" of rim width is all to the outside.
Click to expand...
....and I knew all that, just from the backspace, without calculating anything.....
 
33IMP said:
....and I knew all that, just from the backspace, without calculating anything.....
Click to expand...

Most modern 17/18 wheels list the offset and not the backspace, so knowing how to use both is very handy. It's just a different tool, both have their uses, pros and cons. If you understand what each measurement is telling you then there is a time a place for both specifications.

I honestly don't have to calculate anything most of the time, I know the mm to inch conversions in like 1/4" intervals and the breakdowns of offset to backspace and backspace to offset in my head for a most of the A-body wheel sizes anyway.

If the US would just ditch the archaic Imperial system and adopt SI units all of this would be easier. For most folks it doesn't make much difference in the day to day, and for anyone that does anything with math or science the SI system is about a million times easier to deal with.
 
Maybe you can help blue.
I'm having trouble finding 431.8mm tires for my truck, 45.72 centimeter wheels for my BMW, and 508mm tires for my camaro. Where should I look?
I know where I can get 390s for my fox, I just don't want to spend that much.....
 
33IMP said:
Maybe you can help blue.
I'm having trouble finding 431.8mm tires for my truck, 45.72 centimeter wheels for my BMW, and 508mm tires for my camaro. Where should I look?
I know where I can get 390s for my fox, I just don't want to spend that much.....
Click to expand...

Cute. But tire width is already listed in mm’s as the standard. As is offset. Pretty much all 5x4.5” pattern wheels are cross listed as 5x114.3mm. So diameter and width are still inches, so what? But are they really?

Have you checked to make sure those 18’s on your Beemer are 18.000”? Like, there’s no chance at all that they’re 17.992’s right? Or, you know 457mm :p

It’s moot what you call it anyway, all the machines that make that stuff are digital and can read out and reference in either set of units. We all just take for granted that 18’s are actually 18’s, and if we actually measure them it’s with a tape measure that sure as hell can’t pull off a reading down to .008”. Soooooo
 
I'm not looking for the widest tire as I'm only wanting to make sure they fit. 205/60-15 up front and 225, 235, or 245/60-15's in the rear would work just fine for me. I just want to drive my car until I figure out which rims I really want to go with long term. I plan to 18's all around eventually but just want to drive for this summer. Thanks gentlemen!
 
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