Back space help please!!

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67_Slimer

67_Slimer

Real Hot Rods don't shift themselves.
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OK , I have a 67 dart with wheel vintiques 15" x 8" with a 4.5" backspace in the rears. I found a set of wheels that are 15" x 7" but with a 3.5" backspace. I'm thinking they would fit because although the back space is an inch less , the overall width of the wheel is an inch less as well. I'm not great at this back spacing thing. If anyone can tell me if they would fit I'd appreciate it. Thank you in advance.
 
67_Slimer said:
OK , I have a 67 dart with wheel vintiques 15" x 8" with a 4.5" backspace in the rears. I found a set of wheels that are 15" x 7" but with a 3.5" backspace. I'm thinking they would fit because although the back space is an inch less , the overall width of the wheel is an inch less as well. I'm not great at this back spacing thing. If anyone can tell me if they would fit I'd appreciate it. Thank you in advance.
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That's a tough call. You may have to measure what you have carefully and compare. The 15x7"s with 4.25 backspace barely fit on my 70 Swinger. I can't slide my fingers between the sidewall and the quarter lip. Things are tight. The inside clearance (backspace) is usually not an issue with these cars.
 
If the 15 x 8 with 4.5" BS fit, then the 15 x 7 with 3.5" BS will fit too. The only thing to consider would be the sidewall bulge which should actually improve with the narrower wheel.
 
-Boosted- said:
If the 15 x 8 with 4.5" BS fit, then the 15 x 7 with 3.5" BS will fit too. The only thing to consider would be the sidewall bulge which should actually improve with the narrower wheel.
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O , I didn't think of that , I currently run a 255/60 but I would like to run the same , if the sidewall is too big I'll have to drop down a size.
 
toolmanmike said:
That's a tough call. You may have to measure what you have carefully and compare. The 15x7"s with 4.25 backspace barely fit on my 70 Swinger. I can't slide my fingers between the sidewall and the quarter lip. Things are tight. The inside clearance (backspace) is usually not an issue with these cars.
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My inner quarter lip is shaved off for fitment. That's how I fit the 8 inch. If I'm thinking correctly , the 1 inch less backspace should wash with the 1 inch less width. I'm thinking it should fit the same on the outer and more room inner.
 
I agree with toolman. On a Dart is less room than on the open wheel house on a Duster.
A 75 or 70 series tire will give more room but may not meet what you want in the looks department.
At 3.5 backspace the .75 may mean a tight or rubbing at the outer wheel house.
Most of the factory Mopar 15x7'' have the 4.25 backspace.
Now as you may know no 2 Mopars are the same as tolerances vary side to side.
Example is 2 B body cars, 63 and a 65, swapping wheel tire found no rubbing on the 65 but rubbing on the passenger side of the 63.
 
Darter6 said:
I agree with toolman. On a Dart is less room than on the open wheel house on a Duster.
A 75 or 70 series tire will give more room but may not meet what you want in the looks department.
At 3.5 backspace the .75 may mean a tight or rubbing at the outer wheel house.
Most of the factory Mopar 15x7'' have the 4.25 backspace.
Now as you may know no 2 Mopars are the same as tolerances vary side to side.
Example is 2 B body cars, 63 and a 65, swapping wheel tire found no rubbing on the 65 but rubbing on the passenger side of the 63.
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I see your point but if I'm able to currently fit an 8 inch wheel with a 4.5 bs , doesn't the math work out with a 7" with a 3.5" bs? Again , just going by what I am currently running.
 
Measure the clearance to the wheel lip you have now and subtract 0.5". That's pretty much where your new combo would land.

A 15 x 7 w/ 4" bs would put the wheel in the same position as the 15 x 8 w 4.5" bs you have now. So, by going with 3.5" bs on the 15 x 7, you're essentially moving the wheel and tire outward by 0.5".

Edit - This is a mind bender. I am wrong. the outside face of the wheel will be in the same location.
 
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mopowers said:
Measure the clearance to the wheel lip you have now and subtract 0.5". That's pretty much where your new combo would land.

A 15 x 7 w/ 4" bs would put the wheel in the same position as the 15 x 8 w 4.5" bs you have now. So, by going with 3.5" bs on the 15 x 7, you're essentially moving the wheel and tire outward by 0.5".
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confusing but I'll take your word for it. If that's the case , they won't fit and I won't grab them. Thank you. Just trying to figure out where the other half inch disappeared to lol
 
I prefer offset. that tells you where the wheel mount face is. a 8" rim is 9" wide overall, so 4.5" backspace is the same as 0 offset (the wheel mount face is perfectly centered in the wheel, 4.5" of the wheel outboard, 4.5" of the wheel inboard). a 7" rim is 8" wide overall, so 4" backspace puts the wheel mount face perfectly centered, in the same position as the 8" wheel with 4.5" backspace. a 7" wheel with 3.5" backspace has .5" of negative offset, so the wheel mount face is inboard .5", so the outer edge of the tire will be pushed out .5" compared to what you have now.

Typical muscle car era mopar rims have ~1/4" positive offset (so a typical 7" mopar rim has 4.25" backspace).
 
67_Slimer said:
OK , I have a 67 dart with wheel vintiques 15" x 8" with a 4.5" backspace in the rears. I found a set of wheels that are 15" x 7" but with a 3.5" backspace. I'm thinking they would fit because although the back space is an inch less , the overall width of the wheel is an inch less as well. I'm not great at this back spacing thing. If anyone can tell me if they would fit I'd appreciate it. Thank you in advance.
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See my last post (#5) on your other thread. https://www.forabodiesonly.com/mopar/threads/15-x-8-centerline-auto-drags.580360/#post-1975039131

It should help you understand it all better.
 
67_Slimer said:
OK , I have a 67 dart with wheel vintiques 15" x 8" with a 4.5" backspace in the rears. I found a set of wheels that are 15" x 7" but with a 3.5" backspace. I'm thinking they would fit because although the back space is an inch less , the overall width of the wheel is an inch less as well. I'm not great at this back spacing thing. If anyone can tell me if they would fit I'd appreciate it. Thank you in advance.
Click to expand...
This question is a brain bender. You will loose an inch in width but loose that inch in the backspace. 8" wheels with a 4.5 backspace have 3.5" offset to the front. 7" wide wheels with a 3.5 backspace have 3.5" offset to the front. If your 8's fit so should the 7's. Sorry for the confusion but it can be very confusing.
 
toolmanmike said:
This question is a brain bender. You will loose an inch in width but loose that inch in the backspace. 8" wheels with a 4.5 backspace have 3.5" offset to the front. 7" wide wheels with a 3.5 backspace have 3.5" offset to the front. If your 8's fit so should the 7's. Sorry for the confusion but it can be very confusing.
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Mike,

An 8” wheel with 4.5” Backspacing is a Zero offset wheel.
7” wheel with 3.5” Backspacing has 1/2” negative offset.

It feels like you’re getting mixed up a little. It’s tricky when you start combining the the words backspacing and ‘offset’ in the same conversation.

IMG_5650.jpeg
 
4mulas said:
Mike,

An 8” wheel with 4.5” Backspacing is a Zero offset wheel. T
7” wheel with 3.5” Backspacing has 1/2” negative offset.

It feels like you’re getting mixed ip a little. It’s tricky when you start combining the the words backspacing and ‘offset’ in the same conversation.
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I shouldn't used the word "offset". I should have said front space or what ever. What term is used for the space from the drum surface to the front lip? I guess it would be "front space" although I have never heard it called that. :BangHead: :BangHead:
 
toolmanmike said:
I shouldn't used the word "offset". I should have said front space or what ever. What term is used for the space from the drum surface to the front lip? I guess it would be "front space" although I have never heard it called that. :BangHead: :BangHead:
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Yes, but even then your numbers were off a bit.

A 8"w wheel (9" for the purpose of describing backspacing and forward spacing) that has a backspacing of 4.5 is also 4.5 toward the front of the wheel

A 7" (8 for backspacing purposes) with 3.5 backspacing has 4.5 towards the front of the wheel

I’m pretty sure you know all this stuff and maybe just had a brain fart and need another morning coffee. Lol
 
4mulas said:
Yes, but even then your numbers were off a bit.

A 8"w wheel (9" for the purpose of describing backspacing and forward spacing) that has a backspacing of 4.5 is also 4.5 toward the front of the wheel

A 7" (8 for backspacing purposes) with 3.5 backspacing has 4.5 towards the front of the wheel

I’m pretty sure you know all this stuff and maybe just had a brain fart and need another morning coffee. Lol
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Yep, more coffee, That's what I need. I never use offset, always width and backspace. Also the industry standard for offset is in Millimeters. WTF were they thinking.
 
It's already been covered, but this isn't hard at all.

If you use the 15x7" with a 3.5" backspace the wheel will move a 1/2" toward the quarter compared to the 15x8 with 4.5" of backspace. So, at worst you need to make sure you have an extra 1/2" to the quarter to give up with your current combination.

It probably won't be quite the full 1/2" because the tire will be more squeezed on the 7" rim vs the 8", but I wouldn't count on that being a big difference.

If you wanted to use offset, that shows you the difference. A 15x8 with a 4.5" backspace is a 0 offset wheel. A 15x7 with a 3.5" backspace is a -13 mm offset wheel. So the centerline of the wheel moved out a 12 mm (1/2"). If you wanted to keep the tire in the same place, you'd need a 15x7 with a 4" backspace ( 0 offset ).

You can also use a wheel calculator to show you this...

Screenshot 2025-04-15 at 10.00.39 AM.png


Screenshot 2025-04-15 at 10.01.24 AM.png

toolmanmike said:
Yep, more coffee, That's what I need. I never use offset, always width and backspace. Also the industry standard for offset is in Millimeters. WTF were they thinking.
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That only 3 countries in the world don't use the metric system and the other two are Liberia and Myanmar?
 
All of the extra width of the 8" wheel is to the inside, compared to the 7" wheel.
So, the outside fender clearance should be identical between the 7 and 8, with more room to the inside to the spring with the seven.... assuming the same size tire.
So, if the tire on the eight fits, same size tire on the seven will also fit.
 
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