I have been looking at several threads on wheels width vs offset and backspacing. I have found many different answers and many contradictions.
Here is my quest:
- For the same tire size (35x12.5), I want to go with larger wheels (17x9) but would like to have the tire stick out as (17x7.5) stock wheels + 1.5 spacers.
Here are my calculation/assumptions:
- The stock wheels are 6” backspacing or +44mm offset.
- The spacers makes the wheels a 4.5” (6 – 1.5) backspacing or +6mm (44 – 38) (38mm is 1.5”)
- To have the same stick out for the same tire, the wheel center should be the located at the same location under the Jeep.
- Since the offset is measured from the wheel center, I should make sure the larger wheels would have the same offset as the stock wheels + spacer.
- Thus a 17x9 wheel with +6mm offset would be the same as the 17x7.5 + 1.5 spacer wheel.
This web based calculator confirms my calculations.

http://www.rimsntires.com/specs.jsp

Here is my session output:



Now, surprisingly the backspacing of the 17x9 +6mm offset is 5.3”.

Been looking at many thread and the general consensus seem to be that I should make sure I am below 4.75” backspacing to fit 12.5 tires.

So where is the catch?

 

https://www.jk-forum.com/forums/show...6-wheel-offset

 

Thank you for the link. I already did look at it during my quest. It is in-synch with my understanding.

Still, for the same tire, it means a 17x9 wheel with +6mm offset (5.3" backspacing) would give me the same clearance as a stock wheel + 1.5" spacer.

It also means a 17x10 wheel with +6mm offset (5.8" backspacing) would give me the same clearance.

So what is the point about the hard requirement of 4.75" backspacing on a JK? It is only for small wheels (7.5" and 8")?

 

The backspacing is needed to clear the rear swaybar end links and lower control arms on certain height lifts.

 

Several web calculators and my own calculations proves that the tire itself will be located at exactly the same location in the wheel cage. This stick out would be the same. That means the tire will clear.

The only difference is the wheel itself. I do not understand how the wheel would not clear if the tire does.

 

There are other variables here that you aren't taking into consideration. One of them being the width of the rim. The width of the rim will change the width of the tire. By how much depends on the tire. 2 tires of identical size from different manufactures can by different.

4.75" isn't a hard number, it's a guideline that usually works. Depending on the tire and wheel combination you may be able to get by with a little more or a little less.

Your best bet of finding the absolute maximum backspacing that works would be to find someone running the exact tire wheel combination that you want and see how it fits for them.

If you could say X backspacing works for Y size tire on Z sized rim then that information would be available. There's a lot more variables in play, than that.

 

Ok so if I make sure I have enough clearance for the extra tire width due to the larger rim... then I should be fine with my 5.3" backspacing.

Thank you!