Tire & wheel offset question. What am I doing wrong?

[Flip]

I understand back spacing or at least I thought I did, and I thought by using a chart I could at least get an understanding of wheel offset.

When comparing two different wheels with very similar back spacing on the wheel offset calculator. It is showing a very different measurement.

According to the wheel calculator.

1. Original setup - factory wheel
255/80r17 - offset +63

New setup - Black Rock wheel
255/80r17 - offset +25
Equals a difference of 1.77 tire further away.

2. Original setup - factory wheel
255/80r17 - offset +63

New setup - Vision wheel
255/80r17 - offset +40
Equals a difference of .91 tire further away.

1.77 - .91 = a difference of .86"

The back spacing between wheels is 5.48 - 5.3 = a difference of only .18"

1.77 is almost double the amount compared to .91 with only a difference in back spacing of .18" How is this possible? What am I doing wrong?


Wheel info & links

Black Rock wheel
Backspacing: 5.48
Offset: +25
Wheel Diameter: 17
Wheel Width: 8

Vision
Wheel Size: 17x6.5
Rim Diameter: 17
Wheel Width: 6.5
Offset: 40 mm
Back Spacing 5.3

Black Rock 8P 17x8 25 Black | 937787155 | Custom Offsets (customwheeloffset.com)

Vision Wheel SW60 17x6.5 5x127 40mm Black | SW60-7673B40 (vividracing.com)

Wheel Specs | Wheel Specialists, Inc.

Wheel Offset Calculator (tiresize.com)

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[Yankee1019]

Someone else can correct me if I'm wrong but back spacing and wheel offset have different reference points so the wheel width is a big part of the equation.

 

[Bleda2002]

Different rim widths won't allow you to compare offset. Offset is from the mounting surface to the middle of the rim, backspacing is how much it sticks in from the mounting surface to the back of the rim

 

[azwjowner]

Backspacing is confusing and is a complex measurement that relies upon both offset and rim/wheel thickness. It's also problematic because it's traditionally calculated based on rim backspacing, but if you mount a wider tire on the same rim, backspacing doesn't change yet you'll have less clearance between the inside edge of the tire and your vehicle.

I suggest you use only offset (which is the more modern measurement). If you change tire width, then add/subtract half of that width to the change in offset to figure out how far the inside/outside edge of the tire is moving in or out.

A few issues:

(1) Stock wheels are +44mm offset, not +63mm.
(2) You have Black Rock wheels as +18mm in one place and +25mm in another.

With the same tires, the only difference between how far the tire is moving further away is your offset difference. Tire is 19mm further out if you have +25mm offset wheels. 4mm further out if you have +40mm offset wheels.

 

[CreepyCrepe]

These two guys have it right. Offset and backspacing are similar but different. My understanding is offset is measured from wheel centerline to wheel mounting and backspacing is measured from wheel mounting to the inside rim of the wheel. So you can have a 10in wide wheel and a 6in wide wheel with the same backspacing but radically different offsets and vice versa.

 

[Flip]

Backspacing is confusing and is a complex measurement that relies upon both offset and rim/wheel thickness. It's also problematic because it's traditionally calculated based on rim backspacing, but if you mount a wider tire on the same rim, backspacing doesn't change yet you'll have less clearance between the inside edge of the tire and your vehicle.

I suggest you use only offset (which is the more modern measurement). If you change tire width, then add/subtract half of that width to the change in offset to figure out how far the inside/outside edge of the tire is moving in or out.

A few issues:

(1) Stock wheels are +44mm offset, not +63mm.
(2) You have Black Rock wheels as +18mm in one place and +25mm in another.

With the same tires, the only difference between how far the tire is moving further away is your offset difference. Tire is 19mm further out if you have +25mm offset wheels. 4mm further out if you have +40mm offset wheels.

oops! I corrected that error on the + 18 thank you for pointing that out, it is +25

MY factory 7.5" wheel measures 6.75" of back spacing, looking at the chart under a 7.5" wheel with 6.75" of BS, it shows an offset of +63 ?


Tire is 19mm further out if you have +25mm offset wheels.
4mm further out if you have +40mm offset wheels. ?This one; is an additional 4mm further out from the 19mm +25mm offset? Making it 23mm total, further away out?

I'm so confused, I've been at this for hours.

 

[azwjowner]

oops! I corrected that error on the + 18 thank you for pointing that out, it is +25

MY factory 7.5" wheel measures 6.75" of back spacing, looking at the chart under a 7.5" wheel with 6.75" of BS, it shows an offset of +63 ?


Tire is 19mm further out if you have +25mm offset wheels.
4mm further out if you have +40mm offset wheels. ?This one; is an additional 4mm further out from the 19mm +25mm offset? Making it 23mm total, further away out?

I'm so confused, I've been at this for hours.

Your factory wheel is stamped ET44 (offset +44mm) on it somewhere. Not sure how you're measuring, but I'd forget about backspacing.

If you switch to a +25mm offset wheel, the entire wheel is (44-25=19) 19mm further out. Tire, rim, everything.

If you switch to a +40mm offset wheel, the entire wheel is (44-40=4) 4mm further out. Tire, rim, everything. It's very simple with offset.

 

[CreepyCrepe]

Read this.

 

[Flip]

Your factory wheel is stamped ET44 (offset +44mm) on it somewhere. Not sure how you're measuring, but I'd forget about backspacing.

If you switch to a +25mm offset wheel, the entire wheel is (44-25=19) 19mm further out. Tire, rim, everything.

If you switch to a +40mm offset wheel, the entire wheel is (44-40=4) 4mm further out. Tire, rim, everything. It's very simple with offset.

I measured it like this, and it was 6.75" I will double check it again tomorrow and look for the stamp.

OK, so if it is 63 then it's a difference of 63-40 = 23mm - .9" correct?

Edit: I'm sitting here looking this photo, there is chance that I may have laid the 2' level across the back side of the tire, which would obviously result in an incorrect measurement of 6.75" I will check it again tomorrow.

 

[dlong1119]

Backspacing is from the back edge of the wheel (straight edge placed across it with no tire installed) to the mounting surface. Offset is from the centerline of the wheel to the mounting surface.

Backspacing adds in the approximate 1/2" of the bead lip, whereas offset ignores that and can make things confusing.

I find this to be a much easier calculator to use when comparing wheel setups.
https://marksink.com/tire_wheel_offset/offset.html

For the most part, forget about backspacing these days since everything is generally marked and spec'd by offset. The only time to worry about backspacing is when a lift kit or something specifies a certain amount needed, then worry about checking that using a chart like the one here: https://www.discounttire.com/learn/offset-backspace

 

[Renegade]

It’s so refreshing to see that people have accepted offset as the standard way of comparing wheels these days. When this forum started, the arguments were all over the place, and often flat out wrong. Good job! Carry on.

 

[Bleda2002]

It’s so refreshing to see that people have accepted offset as the standard way of comparing wheels these days. When this forum started, the arguments were all over the place, and often flat out wrong. Good job! Carry on.

If only offset was measured in real units people understand instead of those made up mm things.

 

[SkDmRkLcY]

https://www.wheel-size.com/calc/?wh...80-17X8ET25&fcl=50mm&wcl=30mm&scl=50mm&sr=0mm

This may help or hurt.. in comparison it will speak to backspace and poke under fenders to help visualize a bit.

Backspace you worry about for suspension and component clearance such as sway bar disconnects and it factors by offset and width.

For how far it sticks out only offset is what you are looking at.

if everything is the same width you can assume less offset is also less backspace or vice versa.

Offset on all the factory wheels is 44 I believe.

 

[Flip]

I remeasured my factory wheel, y'all are 100% correct! It is 6" of BS = +44mm offset.

I don't know why the hell I measured it with the level sitting on top of the tire, which is how I came up with 6.75" = +63mm. I'm just not as sharp as I used to be.
This will make a big difference on ordering new wheels.

Thank you to everyone for all of your input and help.

By your deeds I honor you!

 

[Steph1]

I had to go through the offset learning too and what I understand is the lower the number, the more the tire is moved out.

Every 6mm of offset represents 1/4 inch.

Let’s say on your first example, the stock wheel has an offset of +63 and the new wheel of +25

63-25= +38…….. +38/6= 6.33….. 6.33 x1/4= 1.6

So the new wheel is 1.6 more inches towards the outside considering that both wheels are the same width. Otherwise, add the additional width to the 1.6 inches.

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