You're missing an important step in your calculations..... you have to add 1" to the rim width before doing anything.

A 20x9 with 32mm of positive offset actually has 6.26" backspacing.

10"/2 = 5" (backspacing if there was a 0 offset)
Add 32mm of positive offset and you'll come up with 6.26" backspacing.

The 17x7.5 with 44mm of positive offset actually has 5.98" backspacing.

Subtract the differences and you end up with a wheel that sticks out another 1.22".

 

Where does the "mystery" inch come from?

 

It accounts for the rim edge. Look at any wheel manufacturer's website that shows both offset/backspacing and you'll understand. Wheelpros.com is a good one that comes to mind.

 

Um, yeah the wheels you are looking at are 1.5'' wider and the backspacing of the wheels you are looking at is 3/4'' less (or more, however you want to look at it), therefore 3/4'' more it will stick out.

7.5'' to 9'' = 1.5''
5.5'' to 6.25'' = -3/4''
Total = 3/4'' more.

 

He said that his OEM wheels are 7.5" wide and have a +44mm offset (same as my FX4 stockers) and that converts to 6" of backspacing. Add another 1/2" (since it's 6" backspacing and not 5.5") to the 3/4" you came up with and we agree at 1.25".

By the way..... I'm not trying to be a "know-it-all" by any means I've just had lots of experience with offset/backspacing on a previous truck. My last truck had a solid front axle ('06 Dodge 2500 4x4) and offset/backspacing was critical since the axle actually pulls toward the cab during articulation.

 

I was always told that '04-08 stock 17 and 18's had 5.5'' backspacing

 

The problem some folks have when calculating offset and backspacing is the stated rim width (say 20x9 as in the OP's example) is not the same as actual rim width. The stated wheel width is measured from bead seat to bead seat. Most wheels have about a 1/2" lip on either side that must be added to the calculation and that number is used to do the O/S & B/S calculations. A 7.5" rim is 8.5"s overall and a 9" rim is actually 10"s wide overall. Offset is the measurement from the center line of the wheel to the wheel's mounting surface. So a 10" wheel has a centerline of 5" ( 10/2 = 5"). A 32mm (1.26") offset from that point puts the backspacing at 6.26" (5" + 1.26" = 6.26") That is about 1/4" closer to the suspension than stock 7.5 wheels with a 44mm offset & 6" backspace. Depending on what tire goes on the wheel that is probably too close and could cause major rubbing issues. A lower offset such as + 18 would offer a better alternative but would stick out of the wheel well just a bit farther.

 

That's what I've read on here multiple times as well but after checking for myself they actually have 6" backspacing. I know that's correct for my 18" FX4 wheels and I would think that other OEM wheels would be the same or at least extremely close. Some of them could have 5.5" backspacing but like I said I know mine have 6" backspacing/+44mm offset.

Excellent post.

 

This seems to be a common problem on this board when discussing offset & back spacing. It so confused me that I pulled my stock Lariat wheel off my '10 and measured it myself. Can't speak for any other year or wheel type but mine are exactly 6" backspace. That equals 44mm offset on a 7.5" wheel that measures 8.5" at the outer edges of the lip. I suspect but can't be certain that most of the wheels from '04 and up are this configuration.

 

Wheel width is measured from bead to bead, not outside edge to outside edge. The "lip" is not considered part of the rims width. It is the industry standard. There are too many variables when including the lip.

And even if the below examples were not the case, when figuring the difference between two different rims and how much one will stick out further than the other it doesn't matter which method is used to measure the two rims as long as both are measured the same and the difference between the two is calculated.

I have been involved in the tire industry for a long time and up until a few years ago used to work as a freelance tire tester for manufacturers including Pirelli, Michelin and Dunlop. I have run tens of thousands of miles around race tracks while working with tire engineers in the pits to test and improve tire designs based on my feedback and data gathered from the data acquisition system. Every manufacturer I ever worked with measured rim width based on the inside of the bead, not the overall rim width.

Here is a diagram from Tire Rack:


Another example from Coker Tire:
http://resources.coker.com/wheel-tec...rim-width.html

How do I measure my rim width?
A common mistake when measuring a rim's width is measuring the absolute width of the wheel.
Measuring Rim Width:
To determine the width of the rim, measure from bead seat to bead seat across the wheel, not from lip to lip. The "Bead Seat" is the part of the wheel where the bead of the tire sits once the tire is inflated.



Another example from Miller Tire:
http://www.millertire.com/FAQs.asp

HOW TO MEASURE RIMS AND WHEELS… Rim width is the distance between the bead flanges (inside to inside, not outside to outside). Diameter is measured from the bead seat to bead seat.

 

I understand all that and I know that rim width is measured from bead seat to bead seat. However if you're only given offset and rim width you still have to add 1" to the rim width to calculate the backspacing.

This information you posted above is still incorrect and misleading info:

 

You're right. Clearly I'm an idiot with no experience and have no idea what I am talking about. From now on I will defer to you on every matter. Thanks for setting me straight.

 

No need to be a smart ***. I'm just stating the facts and I can't help it if you're calculations don't add up. All I'm trying to do is help out.

Just to back up what I'm saying here are two examples I pulled from a wheel manufacturer's website that show both offset and backspacing:
MO9512063218 20" x 10" -18 4.79 6x135
MO9512963218 20" x 9" 18 5.71 6x135


Now if I were to figure these based off the formula in your example above I would end up with 10.71" of backspacing on the first wheel and 8.29" of backspacing on the second wheel. Using your example as a model:

The first wheel: The math is easy. 10"= 254mm. Add the 18mm of offset (since it's a negative offset) and you have 272mm of backspacing. 272mm= 10.71"

The second wheel: The math is easy. 9"= 228.6mm. Subtract the 18mm of offset and you have 210.6mm of backspacing. 210.6mm= 8.29"

The first mistake you're making is you're not working off the centerline of the wheel. Maybe you should look at your own illustrations above......

 

Sorry but this does not compute.

Here is a very informative link that explains the correct way to measure offset & backspace.

http://www.chevellestuff.com/tech/wheel_offset.htm

 

Yep, agreed. Thanks for the supplemental info 08FX4 and FRANK. I've also seen people make mistakes calculating this on this forum. Hopefully William's head isn't spinning.

I linked to this calculator earlier in the thread, and I'll do it again because it came up with the same calculation of 1.22"

http://www.autolounge.net/calculators/tirespacing.html