Now for the fun part: Will it fit? Offsets, backspacing and clearance! So you know what each of them means...
First I need to note that BBS wheels come with "reverse lips", making it easy to think that you have a 1" lip when in all reality you have a .5" lip.
Here is the correct way to measure your lip/shell
Minus .5" for the "bead" which is NOT used in width/offset calculations, however it can become important when determining inner clearance, remember the "bead" sticks out .5" on both the barrel and the lip/shell which can make clearance an issue on your strut/coil over assembly.
Barrels measure the same way here it's 7.5" but minus .5" from the bead = 7"
Don't forget the face is .5" thick as well.
Here's a "not to scale" example of what I mean..
And here is the correct way to recalculate offsets, backspacing etc. This is essential when building modular wheels as changing lips changes the "et" and swapping faces changed the backspacing and offset.
Here is when just adding a new lip:
1″(inch) = cca 25mm (milimeter)
original wheel : BBS RS 001 ET 25 with 1″ lip
new lip : 2″
width increased by : 2″ – 1″ = 1″
new wheel offset : 25mm(original offset) – (25mm(inceased width 1″) / 2) = 13 ==> ET 13
Basically for every 1" more of lip you add, you minus half an inch or 12.5mm from your OE ET.
Now for recalculating ET which is handy when moving faces around, swapping barrels etc.
This can be helpful but I prefer my own math:
Calculating the Offset of a Wheel
First, measure the overall width of the wheel (remember, just because a wheel is 18x7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement from outboard flange to the inboard flange is 7.5” or "bead to bead").
Overall width/2 = Centerline
After determining the centerline, measure from the mounting pad to the edge of the inboard flange (if the wheel were laying flat on the ground – face up – your measurement would be from the ground to the mounting pad). This is your back spacing.
Centerline – Back Spacing = Offset in Inches
Inches x 25.4 = Offset in mm
These can be applied to measuring the offset of any wheel however, not just modular wheels.