I have a 2008 Supercharged (full sized) that came standard with 20" wheels and 255/50R20 tires. I am in the US so it is NAS and left-hand drive. As far as I know we don't have any "tire police" here so I am free to choose any tire that fits...

Thus far I have chewed thru 2 sets of tires by doing sidewall damage to the tires while driving aired down on the beach.

So a couple of questions:

Assuming no lift kit or other modifications, what tire height can I fit. I am thinking I would like to keep the 255mm width but increase the ratio from 50 to 55 or even greater.

I need to air down to at least 20psi but would prefer as low as 15psi; any recommended tire brands that have a sidewall that can handle this task?

I live on a small island off the coast of the Northeastern US. The max speed limit is 45mph and while we do get rain, it rarely snows.

Plenty of folks run 255/55/20 on the 322, they are the standard fit on the 405 so folks bought the 405 alloys with the tyres and put them on the 322.

If you want to get a taller side wall then you could go smaller on the wheel. There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6

Thanks for that. Any tires you can recommend?

32" seems to be the biggest practical diameter. Even then you might get rubbing at full lock / articulation. Rubbing is increased if you go wider for obvious reasons.

When getting bigger tyres think about what you'll do if (when? ) your EAS packs up and you are on the bump stops. If your tyres are in contact with the wheel arches you're in a world of pain!

Personally I would look at the smallest rims you can fit over your brakes and limit the overall diameter to something a bit less than 32". That way you'll get the deepest sidewall which will give you the most benefit when airing down. Obviously a robust tyre choice with decent side walls is a must. Bish.

2005 110 TD5 CSW
2011 TDV8 Vogue SE (gone)

I am going to order a 18" rim and have the mechanic test fit to my car. Assuming it fits I will have a number of tire choices. If it does not fit, I will go with a 19" wheel which - while not ideal - does allow me more side wall height with the same tire diameter. My air suspension has given out on me in the past so I would like to avoid the bump stop issue if I can.

I am sure the only 18" that will fit over a supercharger brembos is the one made by Compomotive, I may be wrong and you may have different options over there

Pretty sure rosco is correct in his thinking.

Don’t think any L322 18” will fit, though you may find a D4 wheel does but I’m not 100%. There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6