255/45 on 18x8J asteroids definitely have no problems on all four corners
even when driving in snow.

The speedo calibration is now perfect according to various radar
checks.

 

But he is talking after market tires of 8.5J with an ET20, while Astroids have ET33, that's an other 13mm outwards, plus an other 6mm for the wider rim ...

 

The wheel calculator says inside diemension is almost the same as stock (+.06mm) and outside is +32mm wider.
I measured the wells on mine and the 308 can handle that with no problems.

The inside diemension was what worried me. With the asteroid rims, there wasn't much room with the 16/225's.

The new 8.5" rims with the 20EP and the 255/40 tire width make the inside measurements very close to the same as stock.

I'll post before and after pics when the tires come in......

 

I don't want to sound stubborn, and as you ordered your wheels already, there is not much to add to it anyway.
Just want to explain a last time why I have my doubts.

Below picture is my rear tire/wheel combo (275/40R18 ET35), some 1/2 ~ 3/4 compressed.

Going to full compression, the wheel moves in an arc, and it will move in slightly inwards.
But as you see the tire is ballooning, that will offset the extra clearance, which will therefore remain more or less the same.
The space between tire and fender lip at that point is less than 1/2 finger.

For your 255/40R18 ET20, the tire will stick out an other 5mm, and your rim an other 9mm (according the calculator).
These values really depend the tire brand and its tolerances, but it gives an idea.
In my situation, there is no room for this 5mm, considering it is a static value, and tires move around and flex.

Just saying ...