bumping this thread because I cannot for the life of me find this information! I have a base 957 with steel springs as well. Car came with 19" wheels, but I picked up the 18" transsyberia wheels off eBay recently. Hoping to run 265/65/18, I wouldn't mind the 255/55/18 size, but would like my build to run what the transsyberia cars did. I just don't know if air suspension is necessary. everyone who's shared it works have a turbo model and air suspension hahaha

 

You can run an265/65/18 with minimal rubbing. A 2.5 inch lift and 1 inch subframe drop will benefit you from the larger tires but you’ll still rub on the fender liners a bit. You can either trim the lower valence off the truck or heat up the wheel wells and massage then inward

here’s my truck

 

on the 955 from the info i gathered ( i just picked up a 05 S) and i decided to go with 265/60/18 as it is not a trail rig and from everything i have seen you can run the 265/65/18's on the 955 with no issue since it has bigger fender wells then the 957 but your on road driving experience will be a bit different with the 65's as opposed to the 60's

 

This like chart Mike over at Eurowise made should be helpful