When you want to run a wider wheel with the same backspacing, the inner edge of the wheel will come closer to your suspension, eventually hitting the frame, control arms, coilovers or bodywork.

I know Andrew knows what he is talking about but there is a brain fart in there. Running a wider wheel with the same OFFSET will bring the inner edge of the wheel closer to your suspension and possible causing clearance issues.

Using backspacing to calculate the ideal wheel fitment off of a test wheel can be misleading if you don't pay attention to the numbers. I just measured 2 wheels in my garage as an actual example.

The first is the stock gxl 15x6" wheel which is stamped +40. Edge to edge this wheel actually measures 7" wide and has a measured BS of 5.110". Now the misleading part that no one tells you when you are calulating the ideal offset is that you have to take into account the thickness if the wheel lips which are usually very close to .500" on each side.

Here is how it all comes together to calculate BS using the advertised width and offset;

1.0" is = to 25.4mm

convert offset to inches: 40mm divided by 25.4 =1.58"

half of the advertised width is 6 divided by 2 = 3

add the offset 1.58" (+40) to half the advertised width (3) = 4.58"BS

Why doesn't the math add up to what I physically measured the BS to be? Because we didn't account for the lip of the wheel which is roughly .5"

4.58" + .5" = 5.08"BS which is very close to what I measured to be 5.110".

The reverse of this can be used to calculate the offset from BS. Here is the scenario. Using the above wheel as my test wheel I will try and figure out the ideal offset for a wider wheel to use on the front of an fc. My car is in storage and I haven't looked at the fitment in a while so THIS IS ALL SPECULATION. looking at the stock 15x6 wheel with 5.110"BS it looks like I have room for the wheel to come inward another .750" which would add up to 5.860"BS for the new wheel. I need to have a 9" wheel in the front so I can now calculate the desired offset from this.

9"+.5"+.5"(each lip)=10" total width

10" divided in half is 5" which would be the center of the wheel

5.860"BS minus 5.0" = .860" offset

.860 multiplied by 25.4mm = 21.84 offset

So +22 offset is the highest you can use on a 9" wheel. Now since the desired wheel is 3" wider and only .750" of that width is being tucked inwards, that leaves the remaing 2.250" of width to extend outwards towards the fender. This may result in the tire sticking past the fender and requiring clearancing of the fender, a narrower tire, or possibly a rethinking of how wide a wheel you really need.

second example is my 15x8 jeep wheels which are stamped as having an offset of 25.4. 25.4mm is one inch

and half of the width is 4".

4+1=5 + .5(lip) =5.5"BS.

in this case the actual BS is 5.565" but .065" or 1.6mm. which is as close as your going to get and could be the result of your measuring accuracy as well as the fact that the wheel lip is somewhat tapered and difficult to measure so i use .5" as a rough guideline.

hope this helps some of you

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