20" rims on rear of 986 Boxster

You can't install 20" Diameter wheels without some sort of fabrication/alteration. Reason being, the tires will end up touching the lower spring leg.

There are only 2 potential solutions:

1) Go with a narrower 20x7.5" wide front wheel (max) and a 20x9" wide rear wheel (max), for example.

2) A. Swap out your MacPherson struts with a coil-over suspension to increase your inner wheel well space by 2" to 3." The lower spring legs/cups on the MacPherson type struts are about 5" in width. Most coil-overs will be about half of that width, which will allow for wider and taller wheel diameters.

B. And/or Use spacers to help move out the wheels, away from the lower spring legs/cups. When using spacers, you'll need to extend the outer fender as well. To achieve this, you'll need to cut and extend the fenders by either using bolt-on, over-the-fender-flares or welding metal fender flares that custom fit around your wheels and tires.

Although you can get 20" wheels to fit, I recommend never putting a 20" on the front of a 986 Boxster. It will never drive correctly. 19" wheels in the front is the maximum I would ever go. For all around look and performance, 18" diameter wheels are best for the 986 Boxster (front 18"x8.5", rear 18"x10.5").

Another option would be to go with 19"s in the front and 20"s in the rear, staggered.