Are you keeping stock wheels? If so, 20 mm is likely to work in the back, but tire will poke in the front, but not sure with what the section width is on the 21" tire. That said, by lowering, you are now introducing other variables, such as distance between
wheel and tire to fender fender, change in camber etc, to which I cannot speak.
Here's what I calculated manually (with no lowering taken into consideration), with my actual car using level and steel metric ruler, but with 19" wheels that are 8" wide, ET39, and the 235 series stock tires with section width of 10". The 21" stock are 9" wide, ET43, but not sure of the section width of the tire.
- Front wheel to flush: 23 mm
- Front tire to flush: 13 mm
- Rear wheel to flush: 28.5 mm
- Rear tire to flush: 22 mm
Note that the above measurements are based on the widest point of the vehicle...ie. very top of fender opening. Also, of course, this is based on my camber which was within spec but at the higher end as per dealer.
As a result, I figured with a 265 series tire on my 21"
aftermarket wheels, I can go with a ET32 with very slight poke of the tire (which will be wider than the
wheel) in the front. The
wheel manufacturer agreed and spec'd them out with that offset.
