Backspacing and Offset - the never ending headache of wheel shopping

[B5BlueBeast]

Put together some info from my own personal research. Hoping it helps someone else out when they look for wheels.

The relationship between backspacing and offset is based on the wheel's width and how the mounting surface (hub) is positioned in relation to the centerline of the wheel. Here's a breakdown to help you understand and convert between the two:

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Backspacing​

• What It Is: The distance from the mounting surface to the inner edge of the wheel (lip).
• What It Affects: Determines how far the wheel and tire stick into the wheel well toward the suspension and other components.
• Measured In: Inches.

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Offset​

• What It Is: The distance from the mounting surface to the centerline of the wheel.
• What It Affects: Determines how far the wheel sits inward or outward from the hub.
  • Positive Offset: Mounting surface is toward the front of the wheel (closer to the outside).
  • Negative Offset: Mounting surface is toward the back of the wheel (closer to the inside).
• Measured In: Millimeters.

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The Relationship​

The key difference between backspacing and offset lies in the reference point:

• Backspacing uses the inner edge as the reference point.
• Offset uses the centerline of the wheel as the reference point.

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Conversion Formula​

To convert between backspacing and offset, you’ll need:

1. Wheel width (in inches).
2. Backspacing (in inches).
3. Conversion factor: 1 inch = 25.4 mm.

Formula:

1. Find the centerline of the wheel: Centerline = Wheel Width/2
2. Calculate the offset: Offset (mm) = (Backspacing−Centerline) × 25.4

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Example​

Suppose you're looking at a 10-inch wide wheel with a 5.5-inch backspacing (Adjust for your wheel combo):

1. Find the Centerline:
   Centerline = 10/2 = 5 inches
2. Subtract the Centerline from Backspacing:
   5.5 − 5 = 0.5 inches (positive offset)
3. Convert to Millimeters:
   0.5 × 25.4 = 12.7 mm offset (positive)

So, a 10-inch wide wheel with a 5.5-inch backspacing has a +12.7 mm offset.

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Quick Reference Table (Approximate)​

For a 10-inch wide wheel:

• 5.5-inch backspacing = +12 mm offset.
• 5.0-inch backspacing = 0 mm offset.
• 4.5-inch backspacing = -12 mm offset.

 

[Unconventional]

Up until recently SoCal had a couple places that could make what you want with those numbers. Pick the wheel centers you want, pick the rim width, tell them where you want the mounting surface.

 

[toolmanmike]

I prefer backspace. Offset seems to confuse people. Measure what you have and how much space you have left. That will get you very close to what works on your car unless you mess with too big of tires on too narrow of a rim. Like my 255-60's are just about too wide for my 7" wheels. The sidewalls bulge out and are close to rubbing.

 

[1971 CY Dodge Dart]

Like my 255-60's are just about too wide for my 7" wheels. The sidewalls bulge out and are close to rubbing.

So would an 8” wheel with the same size tire get you a little more clearance?

 

[toolmanmike]

So would an 8” wheel with the same size tire get you a little more clearance?

The 7's and 255's are real close. It would have to be a specially built wheel with no guarantee. I don't want to trim or roll the wheer lip.

 

[33IMP]

Not QUITE right.
A ten inch aluminum wheel is really eleven inches wide, from edge to edge.
So a ten inch aluminum wheel with 5.5 backspace is zero offset.
I'm old. I don't care for offset, because it has to be calculated, and you have to know the wheel width. Backspace can be measured, even with a tire on the rim, and not knowing the width!
Tell me wheel width and bs, I'll have a good idea if it'll fit. Offset tells me nothing, cause I don't bother trying to figure out mm or + or -.

 

[33IMP]

So would an 8” wheel with the same size tire get you a little more clearance?

No. Section width of a tire gets wider on a wider rim. That's why when a tire is sized, they mention what the measuring rim is.

 

[33IMP]

Tire size.com says to use a 7-9" rim with a 255/60, so youre good.

 

[toolmanmike]

Tire size.com says to use a 7-9" rim with a 255/60, so youre good.

Yep

 

[72bluNblu]

and it seems offset isn't used by many wheel manufacturers

Not true at all. Offset is used by pretty much ALL wheel manufacturers, and the offset is usually what you'll find stamped or cast into the wheel. For example, this is a 17x9 Legend 5 wheel with a 5" backspace, 0 offset. The "0" stamp didn't come through completely, but under "ET" it has part of a 0 stamped in the box. ET is offset.

Outside of the classic car world, offset it used predominantly. Backspace is easier to measure, but if you know the width of the wheel and the backspace you can just use basic arithmetic to get the offset.

The nice thing about the offset is that it always references the center line of the wheel, so when changing wheel sizes it can be a good reference in keeping the centerline in the same place.

 

[toolmanmike]

Not true at all. Offset is used by pretty much ALL wheel manufacturers, and the offset is usually what you'll find stamped or cast into the wheel. For example, this is a 17x9 Legend 5 wheel with a 5" backspace, 0 offset. The "0" stamp didn't come through completely, but under "ET" it has part of a 0 stamped in the box. ET is offset.

Outside of the classic car world, offset it used predominantly. Backspace is easier to measure, but if you know the width of the wheel and the backspace you can just use basic arithmetic to get the offset.

The nice thing about the offset is that it always references the center line of the wheel, so when changing wheel sizes it can be a good reference in keeping the centerline in the same place.

Thanks for the education.

 

[autoxcuda]

Up until recently SoCal had a couple places that could make what you want with those numbers. Pick the wheel centers you want, pick the rim width, tell them where you want the mounting surface.

Did those place recently go away?

Which ones specifically ?

 

[toolmanmike]

Not true at all. Offset is used by pretty much ALL wheel manufacturers, and the offset is usually what you'll find stamped or cast into the wheel. For example, this is a 17x9 Legend 5 wheel with a 5" backspace, 0 offset. The "0" stamp didn't come through completely, but under "ET" it has part of a 0 stamped in the box. ET is offset.

Outside of the classic car world, offset it used predominantly. Backspace is easier to measure, but if you know the width of the wheel and the backspace you can just use basic arithmetic to get the offset.

The nice thing about the offset is that it always references the center line of the wheel, so when changing wheel sizes it can be a good reference in keeping the centerline in the same place.

For fun I just looked up a Cragar Ss. Backspace was in inches and the offset was in metric. LMFAO

 

[33IMP]

I'm not sure where they were, but wheelsmith is gone, closed their doors, and wheel vintiques got bought by Coker, and their offerings have been cut way down. I honestly don't know if w.v. does any custom wheels any more, but I doubt it.
There used to be a place in Ontario Ca. that would/could do custom steel wheels, but I think they have been gone for a decade.

 

[33IMP]

For fun I just looked up a Cragar Ss. Backspace was in inches and the offset was in metric. LMFAO

That's absolutely normal, now.

 

[72bluNblu]

For fun I just looked up a Cragar Ss. Backspace was in inches and the offset was in metric. LMFAO

Correct, backspace is pretty much always listed in inches and offset is always listed in metric, that's the standard. Given that pretty much the entire world uses metric, you can take a guess as to which one is used more.

 

[Unconventional]

Did those place recently go away?

Which ones specifically ?

Wheel Vintiques, bought by Coker. All they do now is what's displayed in their catalog, no customs.
Wheelsmith is the other and looks like they will build just about any steelie you want, including wires.

 

[1971 CY Dodge Dart]

The 7's and 255's are real close. It would have to be a specially built wheel with no guarantee. I don't want to trim or roll the wheer lip.

View attachment 1716339419

I want to run a 255/60/15” tire on the back of my 71 Dart like you. I currently have a 235/40-18 back there with a 18x8” wheel with a backspace of 4.5” and like you it’s right up to the fender lip especially on the driver side. But I do have a good inch and a half more space to the springs.

 

[1971 CY Dodge Dart]

This Dart runs a 255/60-15” out back with a 15x8” wheel an and I don’t think any modifications were done to it.
Gen III Hemi-powered 1970 Dodge Dart Swinger

 

[go-fish]

I got a quote for a custom offset rallye here Weldcraft Wheels $175 per


And for 3 piece wheels here
VR Wheels: Home of the Three-piece Wheel Industry Specialists

 

[MV8]

I remember Stockton Wheel being one place that would make wheels to spec based on customer supplied centers, but any fab shop should be able to cut and swap hoops with a jig or hub to spin on and mag-base dial indicators for adjusting lateral and radial run out. Hoops and centers can be purchased from racing wheel mfgs like Diamond.

One could make a simple, adjustable jig to hold a tire on a hub without the rim, get it to the best spot, then measure the jig for the wheel placement. Might buy three sizes to check, then return two when buying the other three. Verify return policy first.

 

[33IMP]

Wow, looks like I screwed up!
Looks like Wheelsmith is still in business. I guess I was thinking of Stockton wheel.
My apologies if I misdirected anyone.

 

[33IMP]

I want to run a 255/60/15” tire on the back of my 71 Dart like you. I currently have a 235/40-18 back there with a 18x8” wheel with a backspace of 4.5” and like you it’s right up to the fender lip especially on the driver side. But I do have a good inch and a half more space to the springs.

Check how close you are to the inner side of the wheel tub (sometimes the tub is closer than the spring) but it looks like you just need a 15x8 or 8 1/2 wheel with more backspace. Maybe 5-5 1/2"
Summit's site will let you search wheels by width and backspace.

 

[33IMP]

I want to run a 255/60/15” tire on the back of my 71 Dart like you. I have a 235/40-18 back there with a 18x8” wheel with a backspace of 4.5” and like you it’s right up to the fender lip especially on the driver side. But I do have a good inch and a half more space to the springs.

If you don't have to have a 15"wheel, check out late model mustang wheels (maybe torque thrust M for mustang) . They have the 5 on 4.5 bolt pattern and more backspace than the typical musclecar wheels. 17x7 or 8.

 

[1971 CY Dodge Dart]

If you don't have to have a 15"wheel, check out late model mustang wheels (maybe torque thrust M for mustang) . They have the 5 on 4.5 bolt pattern and more backspace than the typical musclecar wheels. 17x7 or 8.

I kind of have my heart set on 15” Weld Prostars like the blue Dart in the link I posted.