An inch is 25.4mm, so here are a couple rules of thumb:
- An inch wider wheel at the same offset means adding ~12.5mm to the inside & outside (but if you also go up to a much meatier tire, it'll be more)
- Adding 12.5mm to your offset will shove your wheel back inward 1/2"
If you have stock 19s, your 20mm spacer is:
- Turning your stock 8" +34 fronts into 8" +14 offset, and
- Turning your stock 8.5" +46.5 rears into 8.5" +26.5 offset
So for the fronts, if we go from 8" to 8.5" at +34 we'll have 1/4" (6.25m) extra wheel on the outside, vs. the 0.8" (20mm) we want. Reduce offset by ~14mm and an 8.5" +20 should get us there. As a check, going from 8" +14 (what we have with the spacer) to 8.5" +14 has 1/4" or 6.25mm extra outside, which we correct by increasing offset to +20.
For the rears, if we go from 8.5" to 9" at +46.5 we'll again be about 14mm inward of where we want to be, so 9" +32 should get us there. As a check, going from 8.5" +26.5 (with spacer) to 9" +26.5 would leave the same 1/4" or 6.25mm hanging out, so 9" +32-33.
If you want to go a full inch wider, you'll again be adding another 1/4" or 6.25mm inside and out which you can correct by increasing offset that much. So I believe that would be about 9" +26 front and 9.5" +40 rear.
If someone with more experience in Stinger-specific wheels could check my math, that would be appreciated...
