Back space

[Ashman]

I recently read through a thread that talked about the biggest rim and tire you could put on a 69 Dodge Coronet. From what I could gather an 8-in rim with a 4.5 offset seem to be recommended. However when I went into order my new rims the ones I wanted only we're available with a 4-in offset. Can anyone help with will that size still be able to accommodate a nice Wide Tire in the back without any rubbing issues? Any help on this is really appreciated.

 

[440beep]

On my 69 Coronet Super Bee, I’m running a 15x10 wheel with 5.5” backspace. Tires are 275/60-15 and everything fits. Tires are about a 1/2” from leaf spring and inner fender lip.

 

[33 IMP]

I recently read through a thread that talked about the biggest rim and tire you could put on a 69 Dodge Coronet. From what I could gather an 8-in rim with a 4.5 offset seem to be recommended. However when I went into order my new rims the ones I wanted only we're available with a 4-in offset. Can anyone help with will that size still be able to accommodate a nice Wide Tire in the back without any rubbing issues? Any help on this is really appreciated.

You are confusing OFFSET with BACKSPACE.
Offset is how much the mounting face of the wheel is off of center.
Backspace is the distance from the mounting face to the inside edge of the wheel.
For example, let's use the wheel that is likely to fit your car on the back, assuming a reasonable tire size. 15x8 aluminum rim, 4 1/2 backspace.
First, it's really 9"wide. Wheel measurement is from bead to bead, so, 8" b to b, plus two 1/2 beads, so 9"
So the backspace (which is MEASURABLE) is 4 1/2 from the mounting face to the inside edge.
The offset (which is ONLY calculable , and only if you know the actual width of the wheel) in this case is ZERO, because the mount face is in the middle of a nine inch wide rim. If this was a 8 1/2 rim (really 9 1/2 wide) offset would be 1/2 (most offsets are measured in that cursed Metric system, so it would be probably called 12 or 13mm)
You can see why 4" of OFFSET certainly won't work. Nobody will build a 8" wheel with 4" offset, because you would either have zero backspace, or 8" of bs (roughly).
I have never bothered to keep track of which direction + or - is. Tell me the width and backspace, and I will have a good idea if it will fit. I can't say that about offset.
440 beeps wheel mentioned above is also zero offset. 5.5"bs on a (really) 11" wide wheel.

 

[33 IMP]

I have a 69 rr with the lip rolled, and a 10.5x28 slick fits fine, on the right rim. Equivalent street tire would be a 295-55x15. If your lip isn't rolled, that might be pushing it.
275 on a 15x8/8.5 rim with 4.5 backspace will likely fit.

 

[Ashman]

You are confusing OFFSET with BACKSPACE.
Offset is how much the mounting face of the wheel is off of center.
Backspace is the distance from the mounting face to the inside edge of the wheel.
For example, let's use the wheel that is likely to fit your car on the back, assuming a reasonable tire size. 15x8 aluminum rim, 4 1/2 backspace.
First, it's really 9"wide. Wheel measurement is from bead to bead, so, 8" b to b, plus two 1/2 beads, so 9"
So the backspace (which is MEASURABLE) is 4 1/2 from the mounting face to the inside edge.
The offset (which is ONLY calculable , and only if you know the actual width of the wheel) in this case is ZERO, because the mount face is in the middle of a nine inch wide rim. If this was a 8 1/2 rim (really 9 1/2 wide) offset would be 1/2 (most offsets are measured in that cursed Metric system, so it would be probably called 12 or 13mm)
You can see why 4" of OFFSET certainly won't work. Nobody will build a 8" wheel with 4" offset, because you would either have zero backspace, or 8" of bs (roughly).
I have never bothered to keep track of which direction + or - is. Tell me the width and backspace, and I will have a good idea if it will fit. I can't say that about offset.
440 beeps wheel mentioned above is also zero offset. 5.5"bs on a (really) 11" wide wheel.

Ok this makes alot of sense thank you. I don't have the rims yet just ordered but they are sold as American Racing 8 inch rims with 4 inch backspace.

 

[33 IMP]

Ok this makes alot of sense thank you. I don't have the rims yet just ordered but they are sold as American Racing 8 inch rims with 4 inch backspace.

That 4"bs means that the wheel will be 1/2 closer to the fender, and 1/2" farther from the spring.
I have my street tires on a 15x8 steelie with 4" bs. What that means is the wheel with 4 1/2 bs can fit a 295 (equivalent) tire, but the wrong rim (4"bs) can only use a 255, on the same car, cause the tire is too close to the fender.
The 255 is about 1 1/2" narrower than a 295, cause the backspace isn't right.
I think you will find a 4 1/2 to 4 3/4 bs will fit better than a 4"bs.

 

[Ashman]

That 4"bs means that the wheel will be 1/2 closer to the fender, and 1/2" farther from the spring.
I have my street tires on a 15x8 steelie with 4" bs. What that means is the wheel with 4 1/2 bs can fit a 295 (equivalent) tire, but the wrong rim (4"bs) can only use a 255, on the same car, cause the tire is too close to the fender.
The 255 is about 1 1/2" narrower than a 295, cause the backspace isn't right.
I think you will find a 4 1/2 to 4 3/4 bs will fit better than a 4"bs.

Thank you very much

 

[not so famous bob]

That 4"bs means that the wheel will be 1/2 closer to the fender, and 1/2" farther from the spring.
I have my street tires on a 15x8 steelie with 4" bs. What that means is the wheel with 4 1/2 bs can fit a 295 (equivalent) tire, but the wrong rim (4"bs) can only use a 255, on the same car, cause the tire is too close to the fender.
The 255 is about 1 1/2" narrower than a 295, cause the backspace isn't right.
I think you will find a 4 1/2 to 4 3/4 bs will fit better than a 4"bs.

I always thot backspace was from the inside mounting edge (bead) to the mounting surface . I need a 15x 8 1/2'' wheel , with 3 3/4'' from the back inside bead to the mounting surface/ my way , is I incorrect ?? LOL

 

[33 IMP]

I always thot backspace was from the inside mounting edge (bead) to the mounting surface . I need a 15x 8 1/2'' wheel , with 3 3/4'' from the back inside bead to the mounting surface/ my way , is I incorrect ?? LOL

Yes, if you are talking about from the inside of the inside bead, to the mounting face. That would be kinda hard to measure. That's why I like backspace , measurement is easy. (as opposed to offset, which CANT be measured)
Inside edge of the wheel, with a straightedge, measure to the mount face. That's your backspace.
Figure your 3 3/4 from inside bead to mount face, add the thickness of your bead (probably 1/2 inch for an aluminum rim), and you get 4 1/4, a fairly common bs. I'd bet a 4 1/2 bs would work for you too. If not, too much bs can be fixed (and perfected!) with a thin wheel spacer. Just make sure if you use a spacer, get a GOOD one. (Billet).

 

[not so famous bob]

Yes, if you are talking about from the inside of the inside bead, to the mounting face. That would be kinda hard to measure. That's why I like backspace , measurement is easy. (as opposed to offset, which CANT be measured)
Inside edge of the wheel, with a straightedge, measure to the mount face. That's your backspace.
Figure your 3 3/4 from inside bead to mount face, add the thickness of your bead (probably 1/2 inch for an aluminum rim), and you get 4 1/4, a fairly common bs. I'd bet a 4 1/2 bs would work for you too. If not, too much bs can be fixed (and perfected!) with a thin wheel spacer. Just make sure if you use a spacer, get a GOOD one. (Billet).

I kinda loaded this question , I have been measuring the backspace from the very outside of the wheel , to the mounting surface , "just wanted to make sure I was right ."
I do need a 15x8 1/2'' wheel with 3 3/4'' back space measured that way , have them on the back of my 68 fastback , but can not find another for my spare 295 65 d15 tire .
Thanks for the confirmation ...