Hello forum folks,
I've currently got 20x10 with -18mm offset and tires are 33 x 12.50 x 20.
Tires stick out 3" beyond fender.
Wheels are out approx 2.375 beyond fender.

I'm looking to get a new set of wheels. I would like to not stick out as far as they do currently.

Is it as simple as sum of offset and wheel width?

Example:
If I had a 20x9 with a +18mm offset would those stick out 3" (current value) - 1" (current rim at 10" minus 1 for new rim at 9") - 18mm (positive offset moving it inward) = 1.7"

Or is this some made up math?

Thanks for your help!

 

this info might help too:
2013 F-150 2wd STX
4.5" lift

 

To figure out how far new wheels will stick out, you can use this method:

Calculate the difference in wheel width – New wheel width (9") minus current wheel width (10") = 1". This means the new wheels will be 1" closer to the fender.

Offset change – Changing from -18mm offset to +18mm offset will move the wheels inward by 36mm (18mm positive offset minus -18mm current offset). Convert 36mm to inches (1.42").

Total change – Add the 1" from wheel width and 1.42" from offset change = 2.42" total movement inward.

So, if your current wheels stick out 3", with the new wheels, they should stick out 3" - 2.42" = 0.58".

Your math is mostly right. Just make sure to double-check all measurements and conversions for accuracy.